I think they're fine either way.
The thing about having their own system is that they have a captive audience. They can sell stuff like Star Fox because it's on a system with few games and a lot of people who buy anything with "Nintendo" written on it. Imagine Star Fox in the crowded market-place of the PS4/XO. People wouldn't buy that last game because they would have a lot more options. After awhile, that "buy everything from Nintendo" attitude would fade away.
The big stuff would still sell very well, of course, but the weaker stuff would be dust in the wind.
A third-party Nintendo would cut down on the amount of content they produce by a significant amount. That's not much of a negative, to be honest, especially considering some of the weaker games the Wii U had on offer.
I don't think they need it, so I agree they are fine either way, but I think they should.
I agree, some franchises would now face competition, but the boost in people able ti buy their games should more than outweight it in the big picture, it won't be "the only one I can get" anymore but will be a brand famous company game that now have 5x the amount of possible consumers to gice it a try.