I mean they went from 95% marketshare with the NES/Famicom down to like 60% with the SNES and then eventually into a total tail spin ending up around 11% by the time the GameCube ended its run ... that's a pretty whopping drop.

The 3DS selling only half of the DS is still a pretty large dud, the XBox One is going to sell about half the XBox 360 ... you see any XBox fanboys really cheering that as a big success? If the PS5 sells only 65 million units, about half of what the PS4 should finish around, you can bet your ass a lot of people were term that a big disappointment. PS5 needs to sell a minimum of 90 million units I would say and a even flat 90 would be a bit of a disappointment. 

Losing 80+ million customers from one cycle to another is never a good thing, especially when the 3DS required a massive panic price cut that led Nintendo to lose a lot of money. Almost every time Nintendo's had a huge success (NES, Wii, Game Boy, DS) the successor has sold notably less, in some cases disastrously less, the GBA being probably the sole exception there though since it had its lifecycle cut short prematurely it's not as well remembered.  

So sure that's probably where that reputation of having doubts about Nintendo's ability to transition success from one generation to another comes from. You can claim its unfair, but you can't really control other people's opinions. If you're someone who claims loudly they're never late, but you're actually late half the time, you can't really sit there and throw a fit when people maybe make a joke about your punctuality. 

Or if a hockey player scores 100 points one season, and then only 50 the next season or a basketball player goes from averaging 25 points per game to only 13 points per game, people are going to ask what's up with the drop, even if 50 points or 13 ppg is still OK for a lot of players. If the iPhone starts to sell half of what it used to sell, that would be a big story. That's just how the world works. 

SNES dropped ~20% from NES then N64 and GC dropped another ~33% each. It's a gradual, continued decline due to Nintendo following the conventional 'more graphics/bigger games' philosophy. None of them are 'huge mistakes after massive successes'.

Whatever measure you use for 3DS, it does infinitely better than the PSV and the PSV's successor. Had Nintendo put out a lower end system like DS then it might have left a bigger opening for PSV given the conventional thinking was that smartphones will swallow up the handheld cazualz like they did with Wii.

Nintendo went to great lengths to explain in detail what their strategy was for Wii and DS and why they expected it to be successful and copied. Putting it down to luck/a fluke/lightening in a bottle alongside NES, GB and Switch is ridiculous.

Take it up with people who have the viewpoint then, there's a lot of them, I'm just telling you why some people view it that way. A "gradual" decline doesn't really change the fact that it was a steep drop overall. The fact that it was multiple generations worth of mistakes that took them from 95% market share to 11% looks probably just as bad because it indicates they kept making a litany of errors each successive generation. 

Losing 50% of your customer base is never really going to be considered a "win", if the PS5 loses half the PS4 market base, people will call it a disappointment, some will call it a failure. 

Smartphones do dominate the casual market by a monstrous amount at that, Switch is successful precisely because it offers experiences deeper/bigger/more console like than the smartphone and got away from the sinking DS/3DS format of being too small scale of an experience. If Nintendo tried to release a Switch with more casual type experiences as the main killer apps and lower specs they would be in a lot of trouble right now. 

The top 10 selling Switch games are very different from the Wii or DS, the Wii's top 10 was dominated by stuff like Wii Sports, Wii Play, Wii Fit, that is not the case for the Switch. Ring Fit is a nice hit buoyed by the fact that gyms are closed or a no-go for hundreds of millions of people right now, but it's not going to be one of the 5 best selling Switch games like Wii Fit was for Wii, not even close. 1,2 Switch is not going to be anywhere close to the top 15 Switch sellers the way a Wii Play was, or Nintendogs for DS. All the IP that are selling are long established Nintendo IP like Mario, Mario Kart, Zelda, Animal Crossing, Pokemon, the difference is those are "big brother console" versions of those IP and not the tiny weeny portable versions but because of advances can be played portably too. That puts the experience on a different level from other systems including Nintendo's own past handhelds. 

Your Game Boy circa 1996 sure as heck wasn't going to play Super Mario 64. The 3DS wouldn't be able to run anything close to Breath of the Wild or even Splatoon would be a large problem (paint physics). It's just a matter of technology being available now that was impossible in the past and third gen 3D ages better than 1st or 2nd gen 3D. To not recognize as playing a role in the Switch's success is also naive. Once you get into the range of about a PS3.5, you get a level of visual fidelity that allows for satisfying versions of genres like open world games, shooters, action games, and console quality 3D platformers that simply were too compromised in the past, even on Vita. A Vita would have to make massive sacrifices to run a game like Breath of the Wild or DOOM Eternal or NBA 2K or Witcher 3 or Mario Kart 8 or Skyrim or Splatoon or Dragon Quest XI or even Mario Odyssey ... 3DS ... forget it. 

The Switch is the first portable 3D console which runs popular modern genre games and you don't feel always like the game/genre has to be completely compromised. 

Food for thought.

In 2022, the Switch will be passing five years old, yet we already know of a few games that can likely pass 5 million sales likely being released that year: Pokemon Legends, Splatoon 3, and presumably Breath of the Wild 2. Each of these could feasibly pass 10 million sales.

How many Nintendo platforms can you think of that got multiple big games in their sixth year on the market? Not even games that sold 5+ million copies, just pretty big games for their time and context? Because personally, I'm only thinking of the NES (Dragon Quest 4, Dr Mario, and Final Fantasy 3 all in 1990) and the Game Boy (in 1999 the west had Pokemon Yellow and Super Mario Bros DX in quick succession). Even other systems that had big titles later in their lives only had these big titles as the only big game released that year (typically a new Pokemon entry).

Much as I have sometimes been unsatisfied with Nintendo's Switch lineup, I can't deny that they've done a great job of pacing themselves out.

They don't miss every second shot.

8/12 Nintendo systems have been successful and outsold their rivals. Their wins happen because they are well planned and executed products, not because of blind luck or flukes or any such BS.

The recency bias of the Wii U and 3DS losing a shit ton of users from their predecessors is likely a stink that sticks with Nintendo yet in the eyes of some people. They will have to successfully show a successful transition to Switch 2 to dispell that in the eyes of some. Switch 2 needs to be as successful as the Switch or at least close to it. That's just how the cookie crumbles, don't want to be in that situation, next time execute better then. Simple as that. 

That's the thing though, even if Switch 2 is a hit, I'm sure it'll just be some new excuse, some new asterisk/disclaimer. 

The problem with N64 imo it is far from just a dropping in userbase, they lose their piece of market share for good and never reclaimed the same space. Nintendo now releases systems for another market space, even if both market spaces antes the same customers they don't compete directly with Sony and Microsoft, that's the only moment in Nintendo history where they really lose a fight

Game Cube was more like a victimn of business decisions from N64, decisions that made Sony take a market share that used to be Nintendo's. Remember at this point Sega, Sony and Nintendo were essentially disputing the same share of customers is most of continents, the market expansion started with Sony making big steps in Europe and Latin America bul ultimately until that time if you want to sell a piece of hardware you need to fight with another console maker. Worse Ninty was losing sales each gen in a market that was getting bigger, red alarm ringing

Wii U bombed hard but damage was not lasting, partially because of Switch strategy make Ninty recovery fast enough to consolidate their unique position as hardware maker, Switch has an appeal that Sony and Microsoft can't replicate

That's why I see N64 as Nintendo's biggest mistake, even worse than Wii U. Wii U bombed, but damage was not lasting. N64 bombed and pretty much "trapped" Nintendo home consoles in a position that their proposition has a very distinct value (Wii and Switch) or they are doomed (GC, Wii U)

About the rest of your post, I don't share the same instances of fellow Soundwave so I'm just ignoring it 

N64 jump of the gun to 3d realms and forget all 2d classics/arcades types Nintendo has launched. One CD-based and 2d/3d ecosystem, in time, is a better choice than a cartridge focus in 3d games only. The N64, besides greats games, suffers a lack of variety and quantity of your library. Plus, Nintendo forget your ethos: arcade base/2d games.

3DS has this but fixed it because Nintendo portables always have the arcade/2d games besides 3d technology. Nintendo lost all the children with 3ds glass free

But 3ds is not failed. N64/GC/Wiiu is all missteps.

Yeah, to be clear I'm not saying Nintendo hasn't made HUGE mistakes. Hindsight is 20-20 but I don't remember anyone at the time complaining that Mario64, OoT and Goldeneye/PerfectDark was the wrong path for Nintendo.