Thats always been the thing when people bring up Nintendo going second or third party. People use Sega as an example of how it would go poorly but Sega did it as their last resort after everything went wrong. If Nintendo did it now in a financially stable place then they would absolutely have enough money to be a Valve or Naughty Dog who can pick and choose what they release and how long they take on it to make it as polished as possible.
Who knows how the NX will do for Nintendo though, maybe it really will be a massive success (personally I doubt it). Im always of two minds when it comes to this kind of stuff. On a personal level I would prefer Nintendo to go second or third party as I have no interest in buying their hardware anymore but there are the occasional games I want to play. On a games industry level though competition is always better so even if I dont want Nintendo's hardware, it at least keeps the other companies constantly working towards something better. We dont want complacency.