OP is pretty much spot on.
People often forget that units sold isn't the measure of a company's success, anyway. Nintendo, for the most part, have remained an extremely profitable company. They don't sell their consoles at a loss and they make plenty of money off of their published games since they don't have to pay licensing fees to anyone else.
The Wii U's sales haven't exactly gotten huge, but the console is now doing well enough that Nintendo just need to focus on supporting it with some good software. For at least the next year, they've definitely got that covered, and they seem to be building a larger and larger pool of their own development houses as well as nurturing indies, which should keep them sorted for games on the Wii U for a long time.
A successor to the 3DS is probably likely in 2016-2017. I don't think we'll see a follow up to the Wii U until a year or two after that, nor do I think we need it.
