They'd be in serious shit if it failed, and there's no reason to believe anyone wants anything like this. Wii U adds portable properties to a home console and isn't exactly setting the world alight. At the moment, the improving fortunes of the 3DS are likely subsidising to some extent the failings of the Wii U. Nintendo would be mad to put all their eggs in one basket when their reputation in the home console market is worse than ever, all they need to do is manage their resources more appropriately. They tend to have a huge number of handheld releases for a while, followed by a huge number of home console releases, with droughts in between. If they can get a more steady stream of games on both and manage their release dates better (for example, it seems a bit silly to have Donkey Kong Country coming out this year in between Rayman and Mario 3DLand - for my money, it'd be more sensible to shift it slightly to fill a gap in the 2014 release schedule, as Wii U has plenty of games around that time), they should be fine to handle two machines at once.
More likely, should they need to make concessions in the distant future (because their cash reserves mean they can continue their current strategy for some time before becoming even remotely worried) is that they go exclusively handheld. Many people are saying handhelds are dying out, but I think the 3DS' growth suggests there is definitely a market for a dedicated handheld gaming machine, and Nintendo would save money developing for home consoles as well. Personally, I want them to stay in the home console market as I don't buy handhelds.