First, I'm getting a little tired of the rhetoric that "Wii U is underpowered". No, it isn't. It's definitely not as powerful as PS4 and Xbox One, but it's still plenty powerful. I have mine sitting proudly beside my PS4, and you know what? The Wii U gets a lot more play. Moreover, its best games are *drop dead gorgeous*. I don't care how you slice it, a game that is a) fun, b) beautiful and c) exclusive is all the reason anyone needs to buy the system. The age of "some consoles deliver ugly games" is *over*.
Second, while there's no denying the Wii U's first year was pretty rough, it's not the only console to ever have a rough first year. PS3, as you may recall, floundered around for ages before it finally picked up steam. That system didn't really get going until the slim launched almost 3 years later.
So yes, I DO think the Wii U can manage at least 25 million lifetime sales. What it's going to come down to is *games*. It already has the best library of any current generation console, and that's only growing, finally at a pace that's accelerating. Moreover, its recent sales spikes have extended its lead over Xbox One globally. If you want to worry a system might not reach 25 million, maybe you should start worrying about the one that's selling *least*.
