Never nunderestimate Nintendo. They only time they ever "failed" was when they tried to compete directly with the competition with the Gamecube. When they unveiled the Wii people scoffed just as they are now and it absolutely destroyed the competition in terms of sales.
I don't think the Wii U will do quite as well as the Wii, as their focus have shifted slightly back onto the "core" market. But I sincerely doubt it's going to be a failure. Chances are it's going to be a cheaper option than Sony and MS's effort by the time they come out, so we'll see.
My money's on a Sony resurgence this gen. Microsoft look like they're dropping the ball a bit. But Nintendo are going to thrive whatever happens.