@Rol I dunno man. The difference with the TP situation and the Zelda U situation is multiple things.
For one, The Wii was more powerful than the GC (or even the original Xbox), but not ASTOUNDINGLY so, thus it was just a straight GC port, flipped, with slapped on motion controls. The game was otherwise no different, and did not have additional content, nor superior graphics, or anything else that made the Wii version THE version to own. The Wii version sold like it did by virtue of the fact that the Wii sold like it did, on the back of Wii Sports. The GC version still sold around 1.5 million copies itself, and likely would have sold around 4 or 5 mil easy, on GC, if it had remained GC exclusive.
But the point is, with Zelda U, the possibility that the NX is going to be significantly more powerful than Wii U, and thus the fact that at the very least, a Zelda U port to NX might very well have better graphics and thus be the "better version" of the game, at that point you might as well not even release Zelda U on Wii U at all. There would be no real point, because why would anyone want the inferior version? With TP, there was a very strong case to be made that original GC version WAS the superior version of the game, playing it how it meant to be played, non-mirrored, with a regular controller. That is a key difference.
Secondly, the Wii did not have any major titles lined up for the launch window outside of Wii Sports, and Nintendo had absolutely no way of knowing at the time that Wii Sports would blow up like it did. For all they knew, it could have been a dud, only selling because it was a pack-in. So they needed a "big" title for launch, seeing as the GC itself lacked one, so they forced TP over to Wii. It just so happened that Wii Sports blew up, and wound up overshadowing TP, but TP was still initially seen as THE launch title for Wii, releasing a month LATER (which was lame) on GC.
But NX is not going to have that problem. As I've pointed out in other threads, it's already got two "big guns" very likely to be ready for the launch, in the form of whatever the hell Retro is making, and the next main 3D Mario game. That alone is more than enough, and it's always possible (while I think it'll be a Wii U game) that Pikmin 4 will be an NX launch title as well. But, 3D Mario by itself is going to be a system seller. Especially if it takes a more ambitious route than, say, 3D World did. A new epic 3D Mario on more powerful hardware, is going to get people buzzing, that's going to make people want to buy the console. Mario games have ALWAYS been bigger system selling type of titles than Zelda, as popular as the Zelda franchise is. So not only does NX very likely not suffer from NEEDING a Zelda to be ported for launch, but it almost seems (conversely) to be pointless, as 3D Mario the next Retro game will likely be THE big games to get (along with other stuff like Dragon Quest XI etc.)
To MY thinking, as I've been saying all along, NX needs to not come out until 2017. And it needs to have a major 3D Mario game as it's launch window centerpiece. That has to happen. And with that said, Wii U also after 4 years still lacks what I think most people would consider it's "definitive" game. The game you point to and go "THAT is the Wii U". NES had SMB1, SNES had Super Mario World (or DKC), N64 had Mario 64 (or OoT), GC had Melee (or WW). Wii had Mario Galaxy (or Wii Sports). Wii U has........what? What it has are many GOOD games. But it does not have that ONE signature title. And I think Zelda U at least has the CHANCE to be that title. And Wii U, the console itself, deserves that. Longtime Wii U owners, who have been WAITING for such a signature game, deserve that. And NX simply does not in any meaningful way, NEED Zelda U to be ported to it. At all. Wii U NEEDS it as a last great hurrah, a beautiful swan song, FAR more. Wii U both needs and deserves to have a full 5 years on the market, and it should go out strong with a solid lineup of titles...which it's shaping up to have, especially IF Zelda U is permanently Wii U exclusive.
I'm sure you may disagree with me, but still....those are my thoughts on the subject.