Also, "guest" characters seem to stick around in Smash-- unlike, say, Soul Calibur.
Ah yes, using the echo chamber as a measuring stick.
Snake was easily the most missed cut from Brawl to Smash 4-- heck, I knew someone who refused to buy the game until it went on sale because of it. There's a reason he was the one to show that "Everyone is here". Also, a big part of that reason is that (2nd bolded point) 3rd party characters generate far more hype and, through that, revenue for Nintendo. It's a tried and true fact at this point. So, short of only counting the echo chamber, can you really claim "the people who play Smash are primarily Nintendo fans" with a straight face?
The series consistently sells over 10 million, and it's a basic trend for people to own more gaming hardware than just a Nintendo console. Smash has an infinitely broader audience than this online echo chamber, again illustrated by the overall reception to 3rd party characters.
"Moreover, we are getting to a point where the real popular Non-Nintendo characters are drying up. On the other hand, there are a ton of Nintendo characters that have yet to be added and the list will only grow."
...What.
From large (Doom Slayer, an Assassin's Creed character, Monster Hunter, Minecraft, Resident Evil, etc.) to more mid-tier stuff (the Tales series, Undertale, Dynasty Warriors, etc.), there's tons more choices that would excite people far more than the likes of freakin' Dixie Kong or Bandanna Dee. And that's completely ignoring the series already in Smash-- you think people wouldn't be hyped as hell for Shadow, Zero, Alucard, etc. ?
Even B-list 3rd parties are a hell of a lot more interesting and greater hype/revenue generators (just see Joker) than D-list 1st party characters.