Well written OP, the philosophy behind Wii U was the same as Wii, and it has almost nothing in common with Gamecube other than (through 1 year) poor sales. The only thing I would disagree on is that the consumers that bought it have "moved on" to tablets and so on as they always say. Rather, I think there are several issues affecting it:
1. Wii was the first device to make motion controls popular. Very cool. Wii U is a "me too!" tablet. Not much buzz there.
2. Wii launched with Wii Sports (very appealing to non-Nintendo fans) and Zelda (huge core title), and with the awesome sounding Mario Galaxy announced and shown. Wii U had Nintendoland(for Nintendorks only) and MarioU(looks identical to every other Mario game), and 3DWorld only shown later and with a dud reveal. That's what we call a botched software launch.
3. Wii lost a lot of momentum when games ditched it for PS360 later in the gen when HDTVs became popular. Those consumers may look at Wii U as a step back and not forward.
4. I don't think a lot of Wii owners have "left" for tablets, but I do think a lot of new potential gamers are picking up tablets instead. iPads are the hot item for kids, and that means that a lot of the new generation of gamers that Wii picked up over the last decade are not quickly jumping on a game system.
