@OP
N64 had decent third party support, it just didn't get a lot of multiplats (aka PS1 games). But on the other hand it got quite a lot of pretty good third party exclusives.
GC had strong third party support, and there were only a few major examples of multiplat games it didnt' get. It's main problem was the opposite, in that it didn't get nearly as many third party exclusives, though there were a few, like the RE1 remake.
Wii had really strong third party support. It just didn't get a lot of the big multiplat titles like AC and Battlefield, GTA, etc., because those companies were too lazy to downport, even though in many cases the same companeis were perfectly willing to spend the time and resources to downport to DS, to PSP, and even iPhone. They would put SFIV on iPhone, but not Wii. Because that makes total sense. And they wouldn't port MvC3 to Wii, but they made Tatsunoko vs. Capcom for it, a game that was in some ways better. Again, makes perfect sense.
Wii U is the first Nintendo home console to have MAJOR third party issues. Let's be perfectly clear about that. It started out with GREAT support at launch, and if sales for third party games (and the system) had stayed strong, you'd probably still see great support right now. But that didn't happen. Instead, the 2013 drought happened, because Nintendo put too much of their resources into 3DS development, and not enough into Wii U. They have more developers and more teams now, and they've got a better grip on "HD development". But still, it was a problem they should have seen coming.
But this "Nitendo hasn't had good third party support since SNES" is a myth. SNES had GREAT third party support. Fuck, SNES even got a late port of SF Alpha 2. Which just puts more of the lie to their BS over not porting good Capcom fighting games to N64/GC/Wii more often. They are certainly having issues now, because many of the big publishers are losing money, even on other consoles, and aren't willing to "risk" putting their games back on Wii U.
Also, a future where Nintendo becomes Sega, and goes third party? Sorry, but fuck that noise. The reason we get so much great Nintendo software, is BECAUSE they have their own consoles to support. If they went third party, 80%+ of that software would disappear. And they would eventually becoming shitty and a shadow of their former selves, just like Sega is now. People actually want that? Well, then they're not terribly wise.