First, you are stating that some people are supremacist, but, again, this is a Nintendo Fighting game. The game celebrates Nintendo, not other companies. This is even further solidified by the fact that fans expect the third party characters to have a strong relation to Nintendo. Check out every third party character support there here and you'll see reference to how the character is important to Nintendo's history. You can be a "supremacist" if the intent of the game was always fighting with Nintendo characters.
To what you said earlier, you are right. The series isn't too third party-centric as only 7% of the current roster is non-Nintendo characters. But, again, this is a Nintendo fighting game. These characters are guest characters and should be few and far between. Given that, one may argue that 4 characters that make up 7 percent of the roster is still too high.
In regards to quality, the quality of third parties is poor when compared to Nintendo. Nintendo can make more characters and a lot of the characters in Smash 4 were either revived in some way (Palutena, Little Mac) or were new (Rosalina, Wii Fit Trainer, Greninja, Robin, Shulk, Lucina, Dark Pit). Since the game is about Nintendo, any Nintendo character is up for grabs. However, the third parties have always had a higher threshold. One on hand, there are a lot of legal hoops to jump through, so you can add just anyone. And if you are going through all this work, the character needs to be iconic to justify its inclusion. Given the issues, there aren't a lot of good third party characters that will make everyone happen. Basically, there are no more Sonics and Megamans.