The third party support is just going to get worse. We're almost at the point now where Nintendo is the only publisher. So if the Wii U sticks around and gets to the point where there are literally like six games a year how do you think the existing Wii U userbase is going to feel about continuing with Nintendo next time around? And how strong of a mindshare will Nintendo have as a console maker with non-Wii U owners who they logically will be trying to win over? The game releases will become less frequent so there will be a lesser Nintendo presence at stores, less games getting coverage on web sites, less ads in public places. Nintendo as a console maker will be out-of-sight and out-of-mind. They NEED to replace the Wii U or they're just going to fade away further into irrelevance.
Best case scenario: the Wii U is seen as an anomaly and not a typical Nintendo product. Why do you think they killed off the Virtual Boy so quick? They didn't want the Nintendo brand to be associated with subpar products. The longer the Wii U is on the market the more Nintendo will be associated with that kind of unsuccessful product.
You make it sound like the rest of the industry is doing much better when the reality is far from it. We have less retail third party games then ever before because of how high development cost have become. Most studio's are just one game under-performing from being shut down, and under-performing means only selling 3 million copies because you needed to sell 5 million just to break even. For fucks sake, before last gen, most major videogame series would be considered a huge success at 3 million, and many long running series got dozens of installments with sales that never even reached half of that.
Plus there's a good chance Microsoft will sell the Xbox division after this gen considering the One has been a complete disaster for them that's basically lost all money they made off the 360 and reduced their market shares back to original Xbox levels. Yes Microsoft has the money to take losses, but considering shareholders in that company haven't been happy with all the money and resources they've spent on that division, you think they're going to be happy selling less then half of what the 360 after all the money they spent?
Yeah Sony is the only one doing good in the console market right now but the rest of the company is still in terrible shape. What happens if Kaz gets fired because he's unable to turn about the rest of the company and the new President doesn't feel the same way about the games division that Kaz did and decides too makes some changes that don't go over very well.
Point is, Nintendo doesn't need to really rush a new home console out the door since the danger of being irrelevant doesn't mean anything when the rest of the industry is in real danger of collapsing. Nintendo's long term strategy is about just surviving, which they've done for over 100 years now and right now they're the only major company in the videogame industry that has the best chance of still being a part of that industry 10 years from now.