I feel like a lot of this has to do with the casual vs hardcore crowd, and to a lesser extend, and I know some people will hate hearing this, boomers vs zoomers.
Ultimate's sold over 20 million copies by this point as far as I know, I believe the last time they updated the sales rankings it was sitting at over 19 million, and I bet most of those people are either kids, or people who are extremely casual. They don't sit around online and debate the merits of Hayabusa, or conspiracy theory about Geno, or scrutinize leaks, they play Smash when they want to because it's fun.
I mean, Steve broke goddamn Twitter. The silent majority of this community is greater than a lot of people realize, and while the hardcore fans absolutely do get bones thrown their way, Ridley, K.Rool, Banjo, I can't at all blame them for reaching out to arguably the bigger of the two markets. Just because you spend online debating and theorizing about Smash, doesn't make you any different than someone who picks up and plays the game when a new character drops and then waits for the next, and that's probably a majority of the Smash community, even if they take 0 part in discussion online about it.
And as for the other point.... I mean, yeah, it's kinda obvious. Like Minecraft, like Five Nights at Freddy's, like Undertale, people rag on Fortnite cause "Kids", and while I've never played the game myself, my sister does and it seems wholly inoffensive to me, I don't see anything 'wrong' with it at a surface level glance. It's just a video game. I feel like a lot of us older people are stuck in our ways of "Kids these days and their dang default dances and jumpscares! Games were better back in my day!", to which I respond with simply, maybe they were, but we're not living in those times anymore. What's hip with the kids is the future of gaming. You either have to accept that you're living in the past, which, hey, it's not like you'll probably ever run out of games to play, or just, kinda accept that this is what games are like nowadays. Your Minecrafts, your Fortnites, your Fall Guys, your Among Us', your etc etc etc. They definitely still make games for people who are older sure, but you can't blame companies for wanting to cash in on what's hot right now either.
Now, does it have 'staying power'? That, I can't answer, the game is only a few years old, and would be the youngest 3rd party to get in Smash, and by that I mean its IP, Joker is younger but Persona is long running, but I feel like if it 'did' get in Smash, well, that'd definitely help. The game's already big, and if Steve broke Twitter, I legit kinda feel like Jonesy would probably do the same, because again, the casual crowd is huge.
Now, do I 'think' Jonesy, or a Fortnite character is in?.... Not really. I feel like a Mii costume probably, but we also have 0 indication if that leak holds any credibility at all.