Sorry for the late reply
I'd be a bit more optimistic about Dante's popularity remaining if he hadn't already taken a hit in popularity in the post-Ultimate era and if his popularity during the Fighter's Pass era wasn't so closely correlated to DMC 5's release and false leaks relating to Byleth's announcement. A lot of that also has to do with other Capcom characters like the Resident Evil cast and Monster Hunter being seen as viable again and Chun-Li and Zero increasing in popularity due to Sephiroth's inclusion, but he's no longer a Top 5 contender like the fanbase once thought he was.
I agree the Capcom competition will hinder him, but I also think the same will happen for the three (or more) of Microsoft, with one or two pulling ahead of the other. My guess is Chief will pull ahead, Crash will falter and Slayer will be somewhere between them.
The thing about Capcom is I don't think there will be a consistent pack leader. RE has its votes split, and I don't think Chun-Li gets popular enough until we actually see Ryu return - which I don't think will be at the beginning. I know you're including Zero here but he's not going to be as popular as any of these others. And MH probably will be the most expected since it's a unified character in a series that will probably get a new game on Switch 2. However, I think that character, like Hero, veers more towards expected than fan favorite. Outside Japan, at least.
However, were it not the point/topic of this conversation, I wouldn't treat any of these things as foregone conclusions until they actually happened.
I also don't think that Crash or Doom Slayer are necessarily the most comparable characters to Dante, as a lot of their appeal also comes from additional historical value that isn't applicable to Dante. DMC is important, but it was never the PlayStation's "Unofficial Mascot Title" like Crash Bandicoot or completely revolutionizing PC gaming like Doom. I do think Crash is going to be hurt by his franchise's health taking a sudden nose-dive with Crash Team Rumble's commercial and critical failure, but that historical value helps carry these characters during periods of inactivity. I also think there are still a handful of really popular third parties with a notably high Nintendo connection, like Ryu Hayabusa and Geno, so it's not like we're completely out of those woods.
They had historical value, Nintendo appearances, including Nintendo exclusives, and inactivity before Ultimate, and neither got popular until their revivals came to Switch. If Crash and Slayer never got revived, I doubt either would be that popular currently. I can see both of them slipping if the games don't keep up. Historical significance isn't a measure that guarantees enduring popularity. There are both characters who have it without much if any popularity, like Akira Yuki or Chosen Undead, and there are fairly popular characters who really don't have that much, like Geno and Rayman.
But fwiw, I have also seen arguments for Dante on the basis of his significance in codifying the genre of character action. I think all this genre stuff is a little overblown in forming demand and especially in leading to inclusion - it seems more a way to help justify a character who is already popular - but Dante is not without his contribution to gaming's library.
The Nintendo connection really doesn't matter anymore. The only thing it's good for is predisposing characters towards demand. But it's the demand that matters, and demand has clearly moved past the confines of the Nintendo-oriented.
Also, I don't think a DMC5 release on the Switch 2 would sway things much either way.
For popularity, of course it would. That's what we're talking about here. Not chance.
It wasn't DMC5's release that spurred Dante's popularity, it was the first four games coming to Switch (and the leak, and the reduction of competition). It wasn't the revivals that gave Crash and Slayer their popularity nearly as much as it was those games actually coming to Switch. Look at how MH Rise shot that character's popularity right back up - and I also think getting SF6 on Switch 2 would boost Chun Li's popularity.
Now that doesn't correlate to them actually getting in. But we're talking about popularity here. And getting games on Switch does, perhaps erroneously, but nevertheless increase popularity for a lot of characters, at least temporarily.