1.) Of course, I do regard it as a failure. (Although it's not over yet, but I'm not one of the people who are optimistic about Nintendo turning this around.) That's what I was getting at. When you said "4 years absolutely is plenty of time", it sounds like "normal/acceptable lifespan for a console". Personally as a consumer who's used to at least 5-6 years out of my home consoles (My home console history lifespan: 7.5 > 5.5 > 6.5 > 7.0 > ???, and the 5.5 was even one of the failed consoles, N64) I find 4 or less unacceptable for the expectations I have with the money I invested in it, and see it as a sign of failure.

2.) Well that depends on how many games end up beeing exclusive to the N3DS. If for example it ends up just being 3 games, then it's much more of a non issue than compared to hundreds of games.
Looking at a list real quick, out of 41 3DS games released or announced for release after the launch of N3DS, only one of them (Xenoblade) is exclusive to N3DS so far. And that game is of course Nintendo's own game. I think it will be very difficult for Nintendo to find developers who will willingly forsake a 46M (and growing) install base over a completely new one, just for a bit more advanced graphics. I even question how far Nintendo themselves are willing to push it, seeing as the only announced title for it yet isn't even an entierly new game, but a port.

3.) It's not the shared architecture that made me assume relatively weak hardware, but rather the part about Wii U being able to play toned down versions of Fusion games. As we already established, the Wii U is quite a bit weaker than a PS4. So going up to PS4 standards is already about half a generations leap. Going a full generations leap from the Wii U would land it somewhere in between PS4 and PS5. Since I'm already having difficulty seeing how they would make games like that function on the Wii U as well, I'm thinking they wouldn't go much further than that. Leaving it as 'moderately' to 'significantly' more powerful than a PS4. But also significantly less powerful than a PS5 once that one comes out.
It would create a situation similar to how the Wii U was significantly more powerful than its competition, but only for a year. Then it found itself in a reversed role, where it was significatly weaker than its competition. I was hoping Nintendo wouldn't go for being in the middle of the generational leaps again. Especially if the successors to PS4 and XB1 are only a few years away. Though I agree that a PS5/XB2 doesn't seem likely to happen as earlier than 2010 as things stand today. Though I wouldn't rule it out depending on the advancements of technology during that period.
Though of the games you listed, I do think that one game may be pushing the limits of the console, and that's Smash. Not in an obvious way, but the fact that it's running at 1080p 60fps while allowing 8 characters on the screen at once, using only 1GB of RAM.
Though I get your point, as Nintendo rarely push their hardware to the limit, except for on a few occasions. We can expect titles like Zelda U to do that, etc.

As for PS4 and XB1, they are pretty similar in architecture, but some things separate them, such as how they access their RAM. PS4 has a single large unified pool of RAM, while XB1 has to circumvent resources in a more roundabout way to access all of its RAM. Outside of that, it's mostly their specs that separate them.
One difference between PC games and console games though is that console games are locked to run at specific settings, while PC games allow adjustments. If they didn't lock console games to specific settings though, I'd imagine most XB1 and PS4 would be able to run each others games. But the consoles don't allow any discs to start that don't have the correct ID information that identifies it as a legal game for the console. Even if you burn an identical ISO copy of a game and put it in your console, it won't work. (Unless you've modded the console.)
Allowing any disc to run on their system would cause them some unneccesary problems in regards to liscensing and royalties. The Wii was launched without the ability to play DVD's (to watch movies). Or so Nintendo claimed. But as it turns out, it was fully capable of playing DVD's when you use a homebrew program. They simply locked that feature to avoid liscensing fees.

I really hope Nintendo don't give up on making third party developers a big priority for them. I get that it's tough to compete directly with Sony and MS, but constantly betting so much on the appeal of being unique is very risky as well. If they're going to make a relatively powerful console anyway, it seems like a waste to not make the architecture similar to the competition.
And I say relatively powerful, because compared to PS4 and XB1 it might be. Perhaps not when PS5/XB2 come out, but there should be quite a few years before that happens.

I was going to say that it sounds like it would be difficult to force third party developers to always design the game to work properly on 3 different consoles, if they're more interested in making a single game as good as it possibly can be.
I didn't expect you would say that they wouldn't share libraries completely though, nor that missing some features would be ok. I guess that would solve the problem. Just not sure how appealing the Fusion will be if not every game is required to function on all three platforms.

Sorry nintendo, i dont care anymore. I sold my 3ds and wiiu (after a long monster hunter year) to buy a ps4. For me, it was the best decision. I play the major games there, and i havent thought about mario etc. Ever since. And why? I am soooo tired of the 100th mario kart, smash bros or mario plattformer. I still got my n64 and wii. This covers me with the best parts of those series. Only a good zelda would have convinced me. But i am tired of waiting. Maybe they see me if they come up with convincing new ips.

Sorry guys

I expect the next Nintendo console to be announced in 2016 and release in 2017, around 5 years after the Wii U launched. I expect them to want to move onto to next gen sooner than Sony, and possibly MS, which will probably release the PS5 in 2019 or 2020.  (Edit: After thinking about it,  2019 seems more reasonable.  This way they could help promote it with a celebration of 25 Years of Playstation.)

Also, if Nintendo is going to go the cheap route, again, and aim for HW slightly more powerful than the PS4, then they should expect another repeat of this gen. Core gamers, for the most part, are not looking for slightly better graphics with a gimmicky controller. They are going to want a PS5, which will probably be ~4x-6x more powerful than the PS4 and will probably completely blur the line between cinematic and in game graphics.

A few things that pop into mind I'd like to share.

The Cafe (WiiU Codename) was announced only 1 year prior to launch iirc. Nintendo has been reducing the delay between an announcement and the delivery since last gen I've noticed.

The 360 released only 4 years after the original XB, and the GBA launched only 4 years before the DS.

Also, a key point: The successor to the WIiU is also the successor to the 3DS. Why, because the successor is not a hardware configuration, but a software configuration. In essence the next device released is the first of a family of devices which basically share the same games. See my post to Wii-gamer above for the words coming from Iwata himself. There are more such quotes in the subsequent Investor's briefing.

For that reason, the next hardware launch by Nintendo fits with the expectations of a successor to the 3DS. It only happens to be a successor to the U, but people don't know that. In the meantime, the U could still be supported with a handful of games which could be playable on the U and on the successor as the first bridges with the new paradigm.

wii u didn't release too soon and is comparable in power with ps4/xone.