I'd make the argument that the Wii U had several orders of magnitude better 3rd Party Support at launch and shortly after compared to the Switch launch and yet we saw one tank and one skyrocket out of the gate.
I don't think all the 3rd Party Support in the world would have saved Wii U.
Edit: I say this as a huge fan of the Wii U btw
This is correct. The software was there, but the Wii U made absolutely no case for choosing to play that software there and not on some other platform. GamePad features couldn't pull that weight by themselves. You're not going to choose to play ME3 there when you can't carry over all your progress in ME1 and ME2. You're not going to play the Arkham games there just for the GamePad mapping when the DLC support doesn't make parity and when you might played City a year earlier already. You
could make a case for Deus Ex: HR putting its best foot forward as a port (with most of its revisions eventually making their way back to PC), but even so, that's just one game that was a known quantity already in its base form.
If the Switch's third-party situation depended entirely on all the high-profile AAA ports that have shown up—Skyrim, Assassin's Creed (which tanked like crazy on Wii U), The Witcher III, Doom, Borderlands, and the rest—I actually don't think Nintendo would have been any better off than usual, not even with the killer feature of portability (which finally answers the question, "why would you trade performance to play on Nintendo?"). These are ports that presuppose a platform that is already able to stand on its own two feet.
The difference this generation is that the third-party strategy is no longer chasing the staid old console mentality. It's chasing the Steam mentality. So much of the discourse in this forum is still locked into PS2/Xbox/GameCube-era systems war thinking that it's missing the shift in priorities here. The third parties that matter are
here. With the vast majority of those titles, there is little to no performance trade-off at all; the Switch ports are pure convenience, and the answer to the question, "why choose to play on Nintendo?" is straightforward and easy.
When it comes to third parties on Nintendo, you always have to ask that question, and assume that Nintendo will never win on two things: horsepower and online. It turns out the answer is to court different third parties. People can't read this picture accurately if they persist in thinking of indies and small-studio games as a sideshow to the AAA market. Here, as on PC, they
are the market.