I think Wii U will have a good first holiday season and a solid first year with some ups and down, but I'm not sure if this is a platform built for a 5+ year life cycle. I think it will start to fade by about year 3.
Only if Nintendo stops supporting it like they did with the last few years of the Wii. Sure Nintendo could pull another GCN but I highly doubt it. The reason why the GCN didn't sell that much was probably because it didn't differentiate itself from the competition enough. The Wii was weak but different and people bought it in droves.