if Nintendo went 3rd party the quality and polish would suffer since they would not have the advantage of the strengths and weaknesses of their hardware and how to utilize it. Not to mention pressure from the companies they are making games for to but software out quickly rather than efficiently.
Perfect example is look what happened to Sega once they were not relying on Sega inhouse titles to move Sega consoles their polish used to be as good as Nintendo's back in the 90s where games from Sonic Team, AM2 and AM3 were held in the same regard as games from EAD







