Only time I buy switch games is if they are exclusive. Otherwise I would rather have then on PC for the visuals and audio gains.

Yeah, also for the flexibility: 720p or 800p resolution on the Steam Deck, 4K with maxed settings at home, free choice of different controllers or mouse/keyboard, ultra-widescreen-option, ray-tracing option for some games (official support or by mods), VR-option for many games with the UEVR injector, various improvements by other mods 

This is my Switch gamelist:
- The Legend of Zelda Breath of the Wild
- The Legend of Zelda Tears of the Kingdom
- Super Mario Oddysey
- Metroid Dread
- Pokemon Shield
- Xenoblade Chronicles 2
- Super Smash Bros Ultimate
- Luigi's Mansión 3
- The Witcher 3
- Dark Souls Remastered
- BioShock the Collection
- Bayoneta 3
- Shin Megami Tensei V
- Persona 5 Royal
- Nier Autómata
- Doom
- Hollow Knight
- Ori and the Blind Forest
- Ori and the Will of the Wisps

While I agree with most of your post I have doubts about the underlined. I noticed something strange with the WiiU. While pretty much all 3rd-parties abandoned the system, it got pretty much all the platformers. Be it indies or the ones from Sega (does anyone high profile else do platformers). I am not a platformer guy, never was, but it got me thinking. What if I am a fan of good platforming. Chances are I am not too much into PC, because a controller is much preferable to this kind of game and while a PC can be used with platformers it might be a bit more difficult. On Xbox and PS I can find platformers, enough for casually play some, but not if you are deeply into them. Nintendo's systems also have among the 1st party the masterclass in platforming, between Mario, Kirby and Donkey Kong different kinds are covered as well. But if you are done with these, then you will go on to the 3rd-party offerings. Also the power difference of Nintendos machines doesn't matter as much for platformers.

So I have the suspicion, that for many of these platformers from 3rd-parties WiiU was a very good sales platform and Switch can only be better. I don't have proof, but I would assume that to be the case.

It was very true on their home consoles in the Gamecube and Wii U eras. I think this comes from a few reasons.
1. Nintendo first party games are always in extremely high demand on all of their consoles. Most people who buy Nintendo consoles buy Nintendo games, it's a tradition dating back to the NES where people buy a Nintendo console with 1-3 first party games, and maybe a third party game or two. Often the first party game includes killer apps, or the primary piece of software driving the console sales usually to the point where demand outstrips supply by a significant amount: (Major examples include: Super Mario Bros on NES, Tetris and Pokemon on GB, Wii Sports on Wii, Breath of the Wild, Mario Kart, and Animal Crossing on Switch).
2. N64, Gamecube, and Wii U had small numbers of third party games because of various reasons - mainly lower hardware sales.
3. While N64 had a small number of third party games, they were mostly exclusive. Gamecube and Wii U had very few third party exclusives, and often the multiplatforms (particularly on Gamecube) were inferior to those on other consoles: Gamecube controller was insufficient, there were fewer features, and often less content.

But there are other considerations. Nintendo's relationship with third parties is not the same as Sony or Microsoft. Technically, the top selling game on Gamecube is a third party game, but it's considered a first party game because it's published by Nintendo, made by an exclusively (or near exclusively) Nintendo developer, and involved numerous Nintendo licenses. Now, even if only part of that was true, usually those games get counted as first party games (or the colloquial "second party" game, even though technically the user/consumer is the second party). So, it's not really as black and white as it seems.

Scaling because of user base is a thing too. I don't even think the ratio of first party sales to third party sales differ a great deal between consoles (maybe 10-15% more third party on the better selling Nintendo consoles). A lot of it is simply, Wii sold 5 times as many consoles as Gamecube, and therefore sold about 5X as many first and third party games.

The highest selling unambiguously (or by all definitions a) third party game on Gamecube was Sonic 2, 1.73 million (and is the 14th best selling game on Gamecube). The highest on Wii is Just Dance 3 at 10 million (and is the 10th best selling game on Wii) - the numbers are fairly close to the 5:1 ratio for total hardware sales. Surprisingly, the highest selling third party game on Switch is Monster Hunter Rise at 7.7 million units and is the 23rd highest selling Switch game. This doesn't mean third party games are doing worse on Switch because Switch has literally 4 times as many games as the Wii, and nearly 20 times as many when compared to the Gamecube, take into account the points I said earlier about Nintendo owners usually buying first party games, and consider that third party software is fairly dilute on Switch compared to Gamecube and Wii.

PCs support gamepad controllers since 1991... so over 30 years, especially for platformers.

When I bought my first DOS-PC in 1993, of course I also bought my first PC gamepad for platformers & sport games (additional to my first analog joystick for Wing Commander / Strike Commander)

 The Gravis PC GamePad was the first digital controller for PC, which looked a lot like a SNES controller: https://en.wikipedia.org/wiki/Gravis_PC_GamePad

It was compatible to Commander Keen, Zool, Jazz Jackrabbit, Pandemonium and many many other games.

• https://www.howtogeek.com/672716/how-the-gravis-pc-gamepad-transformed-pc-gaming-in-the-90s/
• https://www.stayforever.de/gravis-gamepad-a-conversation-with-ron-haidenger/

In 1996, I got the successor "Gravis GamePad Pro", which added the shape and shoulder buttons of the a digital PS1 controller:

Additional to native game support, all buttons could be mapped with keyboard commands to make almost any PC game (without mandatory mouse controls) compatible.

The Xbox360 controller was PC-compatible from day one: https://www.computerbase.de/2005-11/gamepad-fuer-pc-und-xbox-360-im-handel/

It was so compatible and ergonomic, that the Xbox controller are the standard PC gamepads since 2006 and almost any multiplattform title since then supports it.

And finally Steam added a lot of controllers over the years to their "Steam Input" compatibility:

- 8bitdo SN30 Pro controller
- 8BitDo N30 Pro 2
- 8BitDo SN30 Gamepad
- 8BitDo SN30 Pro+
- 8BitDo Zero 2
- Asus TV500BG Wireless Gaming Controller (Bluetooth)
- BDA PS4 Fightpad
- BDA XB1 Classic Controller
- BDA XB1 Spectra Pro
- 8Bitdo NES30 GamePad in wired mode
- Cideko Air Keyboard Conqueror (PS3) Works fine, but may use up to 3 mins for pairing.
- Cyborg V3 Rumble Pad
- Dragon Shock Generic USB (Moko 3 mde, Wired, USB Dongle and Bluethooth )
- DragonRise Inc. Generic USB Joystick
- GameSir G3w controller
- Gen Game S5
- Google Stadia Controller
- HORI PAD A
- Hori Fighting Commander
- Hyperkin X91
- iBuffalo SNES Controller USB
- iPega 9087s (Red Knight) (Works when using android standard mode with the App.)
- Logitech Wireless Gamepad F310 ???
- Logitech Wireless Gamepad F710
- Logitech G920
- Logitech ChillStream PS3 USB Gamepad
- Mad Catz FightPad PRO
- Microsoft Xbox 360 Wired Controller
- Microsoft Xbox 360 Wireless Controller for Windows (with wireless receiver)
- Microsoft Xbox One Elite Controller
- Microsoft Xbox One Elite Series 2 controller in wired mode
- Microsoft Xbox One Wired Controller
- Microsoft XBox One S Wired USB Controller
- Microsoft XBox One S Wiress Controller (Bluetooth. Use
- Microsoft Xbox One Series X Controller (USB only)
- Microsoft Xbox One Wireless Controller Combat Tech Special Edition
- Microsoft Xbox Duke Controller (Needs remapping on Steam Link. 2 Shoulder buttons missing. USB Adapter Required)
- MOGA XP5-A Plus controller
- MOGA XP5-X Plus controller
- NEXT NES USB Gamepad
- Nintendo Switch Joycons (Needs remapping)
- Nintendo Switch Pro Controller
- Nintendo Wii U Pro Controller
- Nostromo n45 Dual Analog Gamepad
- Nvidia Shield TV Controller (Only works in wired mode)
- NVIDIA Controller v01.04
- OUYA Controller
- PDP Afterglow XBox 360 Controller, (X-Input)
- PDP Afterglow XBox One Wired Controller
- PDP Versus Fighting Pad for PS3
- PDP Battlefield One controller
- Power A Controller
- Power A Batarang USB Xbox 360 Controller
- Power A Mini XBox One wired controller
- PowerA FUSION Pro Controller
- PowerA XB1 Fusion Fight Pad
- Rainbow Force Silver USB
- Razer Atrox Arcade Stick
- Razer Raiju Mobile (Wired)
- Razer Wildcat Controller
- Razer Wolverine Ultimate controller
- Retronic SNES USB Gamepad
- Rock Candy Wired Controller for Xbox One
- Snakebyte Wireless Controller SB904721 with it's USB dongle (It's a PS3 like Controller for the PS3
- Sony DS4 Slim controller, model CUH-ZCT2U
- Sony DS4 Wireless USB Adapter
- Sony PS3 Controller for Wireless use (Must pair over USB cable by pressing the PS button)
- Sony PS4 Controller (Auto pairing using USB Cable)
- Sony PS4 Controller (Wireless, Pair with Home button)
- Sony PS5 Dual Sense
- SpeedLink Thunderstrike Gamepad USB SL-6515-BK (recognized as "DragonRise Inc. Generic USB Joystick" and works in Windows as well)
- Steelseries Stratus XL (Bluetooth, Some buttons mapped wrong)
- SteelSeries Stratus Duo controller
- SZMY-POWER PC Gamepad
- Thrustmaster T150 Racing Wheel (REQUIRES Virtual Here)
- ThrustMaster eSwap PRO Controller
- Thrustmaster TX (REQUIRES Virtual Here)
- Tomee Snes USB Gamepad
- X-Arcade Dual Joystick
- XiaoMi Bluetooth Controller
- ZeroPlus P4 PS4 wired gamepad.

Nintendo fans certainly buy more of them, which I do wonder if third parties consider a problem. Do they consider Sony's first party a problem too, come to think of it?

Of course the statement of 'only buy Nintendo games' may only pertain to AAA games. Wonder if the people who say it would outright say 'Yeah you have 40 Indie games on your system, but you won't buy Mortal Kombat 1, therefore you are the reason AAA doesn't bother with your kind'.

...If Nintendo's large userbase not warranting more focus in development isn't a problem in of itself.

Also gotta understand that some might gel well with the structure of most AAA experiences. Western RPG aren't too much my thing, I'm pass my shooter teenage phase and being open world or extremely realistic might not be the selling points you think could efficiently sell a fanbase that's been conditioned to look for other types of games.

Like personally, I bought so many of Square Enix AA games this generation and other either exclusives or multiplat games that were ported on Switch because these are my style of games I want to experience and you might not see big devs studios seek because they'd be deemed not suitable for mass market. 

As for Indies, I think since the WiiU, Nintendo gave indie devs the access a to wider support with the Indie World presentations and space they've got to share on Nintendo Direct. And it reflex on the sales. You just have to look through the Eshop to realize how many of these games are charting pretty high because people actually seek them. They're not being undervalued by the platform, it's stands proundly in the whole of its ecosystem.

I'm sort of a quasi-lapsed Nintendo fan so maybe I'm not the best one to speak on the matter. Even though GameCube is referenced in my name haha. I guess I'm more of an oldschool Nintendo fan :P

But this was pretty much true for the first half of my life, though in fairness my dad was somewhat strict with gaming and only let me have one console per generation, so I just always picked Nintendo b/c they were, at that time for me anyway, the best. I did play a bit on PC but other home consoles were exclusively the realm of friends and family until I was about 18 years old.

Then that changed in 2004 when I got Dreamcast and 2005 when I got Xbox 360 soon after. Now I play Xbox and third party/indie stuff far more than Nintendo, but still have a Switch, which is my defacto handheld console to play while watching TV. And I play a bit of PC still but mostly that's reserved for RTS and isometric ARPG type stuff like Diablo. The only consistant is I never really got into the PS stuff. 

But going beyond the personal anecdote - I think Nintendo ironically draws from two very distinct but similar groups (in terms of purchasing habits) - devoted hardcore fans that don't expand much beyond their library, AND a more casual/younger/female demographic that ALSO doesn't expand much beyond their stuff, or if they do it's mobile gaming. So their target audience I think sticks more to Nintendo 1st and 2nd party software compared to others. But for my money there are certain genres and styles/traits that they just don't fulfill (RTS, FPS, WRPG, MMO, edgier and flasiher titles) and you still can't find nearly as much on Nintendo consoles compared to others, even with the Switch's massive library. So Nintendo gamers who want to get serious about expanding their horizons will probably want to eventually branch out at least a bit. I know I did .

If you have 50% of your maketshare of SW sold on your system being first party while on PS it is likely below 20% and likely even less on Xbox you can easily see this as being true. Imagine that the 3rd parties make more than 10x more titles than the 1st party so if you need to distribute the same total you will see how poorly in general those titles will sell.
And asking publishers to change their mindset and what they make to entice Nintendo buyers instead of keep doing what they do and is successful for them makes no sense.
You can also look rank of titles on Switch versus the other consoles and see how much platform holder dominates its own system.