Ian, thats assuming that Wii U is successful in all regards, one of the goals of Wii was to get good third party development, and despite this it had initial support that dropped off. So in the eyes of Iwata its for sure a sales success, but not a success with developers. Plus, the Wii is a glorified gamecube..theres only so much you can do with it. Gamecube wasn't a horse that got beat dead, but it did become a zombie.
Also, if you were to look at it from a resource standpoint of course they supported their handheld till the end, they had always good handheld 3rd party support and 2 handhelds basically steal development resources away from consoles. All consoles are fads, not meant to last more than a couple years, the 360 fad and ps3 fad will be over in another 2. If consoles weren't doomed we'd still be playing new NES games, and its not even sure if that one awesome Retro City Rampage is coming out this year.