It would have failed compared to the Wii. The gamepad/tablet is not nearly as compelling to casuals as motion was. I mean especially with touch and two screens already available on their DS, I'd suppose. And at that $400 price... I mean, right now the WiiU is just a DS for your TV most of the time. The gamepad has not been used fully for its mobility/asymmetric play in many games, just HUD/map/menu/drawing.
Also, Nintendo released a console notably more powerful than the PS2 with the GCN, and it still didn't get massive third party support (it still got darn good third party games, but it's no PS2). Getting third party support from publishers would not magically happen if the Wii was the WiiU instead. Nintendo would have to work on business relations, not console specs.
Plus they would have lost money on every console, in significant $50+ amounts. And if they maintained a similar online architecture as they have today for free that would have run them a pretty penny 7 years ago as well. Probably one of the few cost mitigators 360 had for MS early on was pricey online.
Nintendo didn't have a super strong market to build from off GCN. Xbox managed to do it, but look at how much money it took them. And they got many third party games on 360.
