The problem is, you can't forget about the Wii U. It's like the Star Wars prequels, they will never be undone.

Eh it got a 1st party game in it's 5th year. Original Xbox had a shorter lifespan. Wii U got pretty much a typical Nintendo system lifespan. Dreamcast lasted 2 in the US and Wii U sold worse in the time amound of time. Wii U lived pretty long considering.

Not really. The Famicom had Wario's Woods come out 11 years after its original release, the Super Famicom had Thracia 776 9 years after release, Twilight Princess came out 5 years instead of 4 1/3 that the Wii U had.

We keep seeing posts from Nintendo fans denouncing the Wii U as a bad system, an anomaly, with the implication of course that the next console is substantially different and will have a very different fate.

It sounds like the argument is Wii U focused too much on core games. Isn't Switch doing essentially the same?   It's main initial title Zelda is a Wii U game, and it's next big releases MK8 Deluxe, Splatoon 2 look like minor updates to core Wii U games.  It seems somewhat hypocritical to say Switch's software strategy is good and Wii U's bad.

It's really actually the opposite. People keep saying "it had COD and Assassin's Creed" yes ... ok .... SIX YEARS LATE. Let me repeat that .... SIX YEARS LATE. lol. What did they expect, that that audience was going to come running over? Those games were as a result effecively useless, every PS3/360 owner knew PS4/XB1 were imminent anyway with better graphics coming. 

In terms of actual Nintendo games, the Wii U probably had the most casual first year lineup Nintendo has ever made for any system, even moreso than the original Wii. 

New Super Mario Bros. U - 2D Mario heavy crossover appeal for casual market

Nintendo Land - Mini-Game Compilation

Sing Party - Singing/Dancing Game

LEGO City Undercover - Kids licensed game

Game & Wario - Mini-Game Compilation

Pikmin 3 - Traditional Nintendo IP Sequel

The Wonderful 101 - Niche cartoony Japanese game

The Legend of Zelda Wind Waker HD - GameCube game remaster

Wii Party U - Mini-Game Compilation

Wii Fit U - Mini-Game Fitness Compilation

Wii Sports Club - Mini-Game Compilation

Mario & Sonic Sochi Olympics - Mini-Game Compilation

Mario 3D World - 3D Mario platformer w/multiplayer

Even the two "big" core titles for the Wii U in its first year, NSMBU and Mario 3D World clearly had modifications to their game formula to appeal to that "casual" crowd (easy multiplayer). 

And even look at the marketing .... Switch has a grown ass man with facial hair and presumably his own apartment, Wii U is an 8 year old kid playing with mommy and daddy. 

https://www.youtube.com/watch?v=Pl1Dlmf5-nc

Versus

https://www.youtube.com/watch?v=j3XmVt27RI4

Which one of these products is aimed at an adult gamer, and which is a kids/casual focus? It's fairly obvious. 

I saw a Switch trailer before the R-rated film Logan, meanwhile Wii U marketing was primarily on kids cartoon networks. 

I like the Wii U. It's a nice piece of hardware with a bunch of great exclusives. But the marketing was a disaster and the idea wasn't simple nor unique enough to be successful. It lacked third party support, which is objectively a bad thing, but not something I personally care too much about, since I don't have time to play all the first party games anyway. I have had a lot of fun with the Wii U, but it's not among Nintendo's greatest consoles. I would at least put it above the GameCube, which I consider to be Nintendo at their most uninspired and irrelevant. But other than the GameCube the Wii U might be Nintendo's weakest console.

Top 10 reasons why Wii U failed...

#10: Bad name.
#9: Inferior online network compared to XBL / PSN.
#8: Lacking key Nintendo franchises; no Metroid, no true 3D Mario a la 64 or Galaxy, and no Zelda until it was dead already.
#7: Terrible name.
#6: Marketing was horse shit.
#5: "Tablet" controller sucked all around, nothing but a desperate gimmick to try to keep the "casual" audience around that had already moved on to actual mobile devices.
#4: God awful name.
#3: Underpowered and overpriced.
#2: Complete lack of 3rd party support; made N64's and GCN's support look amazing by comparison.
#1: Did I mention how horrendous the name of this console is? Whoever gave that name the green light should be buried in a landfill of Wii Us, Ouyas, Pippins, Gizmondos, N-Gages and Game.coms.

Wii Era in general was weak, I really don't care that soccer moms and fringe gamers thought it was all the rage for about 4 years before they ditched the brand entirely. Mario Galaxy 1/2, Xenoblade, Splatoon, and uh I guess Twilight Princess are nice from that era but not much else stood out as incredible.

80s-90s Nintendo (NES, SNES, N64) are the best Nintendo, hope Switch can regain some of what made those eras work.

It was a fun device, that's for sure. I bought pretty much all the big games for it, as well as some third party games like Deus Ex: HR.

In my book, definitely underrated. Give it 10 years or so, and I think it'll develop a cult following, if it hasn't already.

you also forgot

#11: Lack of software that made compelling use of the gamepad
#12: The price of the gamepad which prevented desperately needed price-cuts.
#13: Overreliance on the dying Wii brand
#14: Terrible software droughts
#15: Overdependence on 2D and party games.
#16: The actual console looks so much like the Wii that the Wii U was easily mistaken as an add-on

It's a wonder the Wii U even sold 14m when you look at the whole picture.  The only reason it was even able to break 10m was because of the strength of Mario Kart 8, Smash 4, Splatoon, Mario Maker and the promise of BotW.  The Wii U was handled so poorly and was such an uncompelling piece of hardware that I really think the majority of its note-worthy exclusives should eventually be ported over to the Switch to give them a second chance, chief among them Bayonetta 2, Xenoblade X, Tropical Freeze, Mario Maker, Smash 4, Mario Kart 8 and Pokken Tournament.