There is no "Nintendo Fusion" yet, it's only speculation on the part of some folks.
But if it DID exist in the future, and was what people propose: a hybrid home console/portable system? It would sell very well. Why? Because it would take the sales their portable system would usually get, ALONG with the sales the home console would usually get. And it would also get all the games, presumably, that their portable systems usually get, including better third party support, since it doubles as a portable console itself. And that includes multi-million-selling mainstream Pokemon.
On the one hand, it would suck, because you wouldn't have two separate systems, with two separate sets of games to look forward to. On the other hand, the hardware would be unified, no two separate sets of specs, and you'd only have to buy one console. The real question would then become, would Nintendo and the many companies under their umbrella, continue to pump out the same quantity of games that they have for, say, Wii U AND 3DS, for this one console? Would it get superb first party support with NO real droughts, ever? Or would Nintendo slow down game production since it's now just one system? If they were smart, they'd do the former, not the latter, so that even if 3rd party support WASN'T there (though I think it would be much stronger), they'd have a very nice flow of Nintendo published games, as well as eShop games.
To me, that is the only MAJOR problem that Wii U is having. Yes, the weak current 3rd party support does suck, even though I don't personally buy most of those games on PS3 either. But the major drawback for Wii U, hasn't been the lack of multi-console ports. It's been the droughts that the system has suffered, months-long stretches where hardly anything, even from Nintendo, has come out for retail. Granted, 2014 wasn't as bad as 2013. But it still had it's lulls, and that shouldn't be the case. Nintendo has enough Wii U games in development, at least a COUPLE more of those should have released at some point in 2014, but they didn't. So I guess the theory is that maybe, if they had on "hybrid" console, it would also get the kinds of games DS/3DS have gotten, and thus wouldn't have those droughts, as Nintendo wouldn't have to split it's development between two systems.
I see the pros and the cons for the idea, honestly. I like the idea of their being a separate portable system, as there has been one since 1989. But on the other hand, only having to buy one system, and that system hopefully getting ALL the games that two normally would, would definitely have it's perks. So who knows....
But it most certainly would sell better than Wii U has been, to answer the OP's post. That doesn't mean that I want this "Fusion" to happen anytime soon, mind you. I personally like and enjoy the Wii U just fine, and think that both it and the 3DS have several more good years ahead of them. As a Wii U owner, who has invested time and money into the system, I want Nintendo to take their sweet time making the next console, and in the meantime pump out as MANY good games as they can for Wii U, because quite frankly, I want my goddamn money's worth, lol. I want this new Zelda, I want a (GOOD) Star Fox. I want a (GOOD, NOT Other M) Metroid game. I want to see more than one future game from Retro. I'd like to see Wii U entries from studios like Next Level and Monster Games. There's tons of potential for great games on Wii U, and I'd like to see at least AT LEAST through 2016, getting continuous support from Nintendo and it's subsidiaries and partners.
So I'm in no hurry at all. If this "Fusion" didn't actually happen until, say, 2017? I'd be totally cool with that.