This.

It's the same game engine, but it's been heavily reworked in order to fully take advantage of the Switch's rendering pipeline.
So some things got lost like the layered alpha fur on Donkey Kong as it's bandwidth intensive and there was no eSRAM/eDRAM, but the more flexible shader pipelines offered far better lighting.

But that doesn't say much. The latest Call of Duty still has engine roots in Quake 2.

The latest Fallout based on Creation still has roots in Gamebryo, which in turn was based on Net Immerse which featured in Morrowind from 2003.

Point is... No one throws out an entire engine and builds a new one from scratch that is just a waste of time and money, rather parts of it are re-engineered and enhanced to meet various needs and design goals.
Remember it's an "engine". - It's a bunch of components working together, not a single entity.

The gameplay styles were quite different between the Gamecube and Wii.  That said, you are right that the hardware was similar.  It is interesting how back then we were never given games like Mario Kart: Double Dash 2, Super Mario Sunshine 2, Wind Waker 2, etc on the Wii.  There was the Twilight Princess re-release but other than this, Nintendo still made the effort to create completely novel experiences and take risks with their main IP instead of building their sequels off of existing assets with relatively minor changes and minimal creative innovation.  Honestly, I don't think that we ever would have accepted it as gamers back then.

I don't think there's anything lazy or creatively inadequate about games like Splatoon 2 or Mario Maker 2 or Smash Ultimate, they're just iterative sequels the likes of which have been a part of the industry forever, including from Nintendo. On the Wii, Mario Kart was an iterative sequel, as was Smash Bros Brawl, Kirby's Return to Dreamland, Mario Galaxy 2, etc.

I am very afraid that Nintendo will repeat many of its mistakes with the Wii U with the Switch's successor. Specifically that they will try to reinvent the wheel again when it's completely unnecessary. The gamepad was innovation for innovation's sake without any thought as to why such a controller might be needed. The changes the Wii and Switch brought were necessary because of how poorly their predecessors did. If they didn't try to be revolutionary and kept the Wiimote format they may not have sold Wii numbers, but it definitely would have sold much better. Heck, the Wii was outselling the Wii U in December 2012 and for much of 2013 despite the Wii being 6 years old with almost no new games because it was still a more appealing system.

I'm really worried that Nintendo will do this again and ruin the good thing they've got going with the Switch by making a huge revolutionary change when all anyone would want is an upgraded Switch that's a generation above the original in horsepower and better joy cons.

Yeah this worries me a lot too. 

All they have to do is make it a standard successor and just call it "Switch 2", it really is that easy, but it's so classically Nintendo to mess up the simplest things and become the architect of their own downfall.

It's the Fisher Price design...

Strongly disagreed on ZombiU.
The game certainly had its faults, especially the ugliness of the graphics, but it used the gamepad in a fun and interesting way and made for a gripping experience. Having to take your eyes off the main screen to dig through your backpack or check your map while the action continued on the main screen was a clever way to induce tension, since a zombie could attack while your character was distracted. It made for a nerve-wracking time where you were afraid to take your eyes off your foes and only checked your gear when you were certain of your safety.
Whatever your criticisms of the game, it certainly wasn't shovelware.

Exactly!

We can certainly have a difference of opinion here.  The Wii wasn't Nintendo at their creatively strongest either, but I have to say that things have gotten worse during the Switch.  The Switch and the Wii are the only two Nintendo home consoles that I have not owned since I started gaming.  I owned a Wii U and with Smash, Splatoon, BotW, Mario Maker, Mario Kart 8 I honestly see no reason to upgrade for the sequels that are on the Switch (let's exclude the fact that the servers and support for the Wii U is gone which essentially forces you to upgrade).  Apart from a new console and some more technical and graphical polish, what is really present in these sequels that I cannot have experienced on the Wii U?  The content differences that are there don't seem to add up to more than what you would expect to find in a few good DLC packs in my view.  Nintendo games already looked great at 720p and with the more animated style of Nintendo games, I do not see 1080p as a big selling feature. 

I will admit that Mario Odyssey is quite tempting and is the one title that I almost want to buy the Switch for, but Splatoon 2, Smash Ultimate, Mario Kart 8 Delux have very little appeal to me because I cannot see anything that is new or really changes how these games are played.  It feels more like a renewal license to continue having support for the games I purchased on the Wii U.  Switch releases like NSMBU and SM3DW only reinforce my opinion here.  Buying Smash Ultimate just to play as Banjo or the inklings just isn't worth the upgrade in my view.  Honestly, the Cloud Strife DLC that the Wii U got was a bigger deal to me than playing as those characters.  The one thing that might get me to eventually buy a Switch would be a new Metroid Prime that really rocks, but that still seems like a long ways away.

This is just my opinion and I am fine if it is an unpopular one.  Don't get me wrong, I am still a huge Nintendo fan and I want the Switch to be successful. That being said, I feel that my future in gaming lies in the past which is why I am designing an adapter that allows me to insert a raspberry Pi into and N64 cartridge slot and use my 64 as a unified platform for retro gaming for games on consoles for the PS1 and earlier.  Buying these games used are getting outrageous, but at least with this you own something physical that does not require constant ongoing support and you can legitimately play your personal backups.  My Wii U now is basically unplayable without support or online and it has some intermittent stability issues booting up that makes me nervous to even turn it on so I can't even play all the Retro eShop and Wii Virtual Console games that I bought on it.

Still, the poor marketing has to take some of the blame.  The Nintendo Switch is a spectacular device, but Nintendo made sure that everyone knew about it by paying for the company's first ever Super Bowl ad.  They didn't just put it out and hope people would notice.  With the Wii U, the marketing and messaging was so bad, that they actually had to put out an ad trying to explain what the thing was... a year after the console had been released!  I don't remember seeing any aggressive TV advertising for the Wii U until it had already been labeled a flop.  The Wii U needed a viral marketing campaign like the "Wii would like to Play" ads. 

And it would have been so simple too.  Just do the same ad where the Nintendo execs show up at your door and say "Wii would like U to play".  Then show off some clips a player using the Gamepad while other players used the screen.  Highlight experiences unique to the Gamepad like the Mario Hide & Seek mini game in Nintendo Land as a draw for kids, and then designing your own hot routes at the line of scrimmage in Madden 13 as a lure for the more hardcore gamers.  And state it very clearly that the system is the successor to the Wii.  It could have been done well and done right, but it wasn't.