I don't really understand why people assume Nintendo would end up like Sega. They are very different companies that would be in very different places if you compared Nintendo of now to Sega of the past.
Personally, I see potential for Nintendo to do well as a third-party, provided they didn't mess it by trying to do something weird (which is possible). As it stands, they occupy a niche that the other publishers aren't very interested in. The average PS4/XO consumer probably isn't going to buy software just because it has Nintendo written on it but it's quite possible that they'll be interested in some of what's on offer. That, and you've got to assume that at least a few hardcore Nintendo fans will grab a different device in order to play Nintendo games.
That being said, I do think so of the hardcore Nintendo-only gamers will discover fun games that they never tried before and over time you'll see less and less of them who feel they have to support every title Nintendo releases. Diffusion will take place.
