To be honest none of the three console manufacturers would be able to adequetely support a modern home console (I mean something that's in the range of a PS4 tech wise or even better) and a portable platform that requires PS3 level assets either.
You look at Sony, this is the THIRD straight holiday season coming for the PS4, and they still haven't managed to internally create a big fall title for the PS4 with Uncharted 4 missing the holiday season.
There's no way any one of these three could manage a decent flow of next-gen games if they also had to make 7-12 handheld games per year with visual fidelity of those games being in the X360/PS3 range.
Expecting Nintendo to somehow be able to do so when they are probably an even smaller company is completely unrealistic. With rising dev costs, Nintendo has to adopt a different model, the fact that the Wii U sold like crap probably just makes it a much, much easier decision.
And I agree with zorg's point above, in fact I'll go further ... the biggest winner from a unified platform approach will be "mid-tier" Nintendo games like Pikmin 3, DKC: Tropical Freeze, Kirby's Rainbow Curse, Captain Toad, etc. All of these games would've likely had sold 2-3x more copies if the 3DS audience had access to those games. But under the current "seperate software ecosystem" setup that Nintendo has, this is impossible, so these games are left to die on the Wii U and underperform.
There's no reason DKC: Tropical Freeze, a sequel in a franchise that routinely averages 5 million per entry should be sucking wind like a fat kid trying to get up stairs just to hit 1 million perhaps. That is a massive dissapointment for an entry in that franchise. Splatoon would also be far bigger if it wasn't stuck on the Wii U only.
The Mario Karts ... they'll benefit too of course, but it's the Pikmins, Splatoons, DKCs, Captain Toads, of the Nintendo world that stand to benefit the most from a unified platform approach, right now these games are getting killed by being left to die on the Wii U's piddly 10 million userbase.
