Niche groups? Have you seen how huge and heterogeneous mastodon.social and the likes are? They're even bigger than that ;-)
Even the "niche" instances have sufficient numbers that there are enough niches for many people to see something of interest there, and most of them freely allow off-topic chatter, nay welcome it.
One of the benefits of the Fediverse is that even if you're on an instance that's for talking about movies, you can a) almost always talk about anything and b) talk to your followers about whatever you please.
The Fediverse isn't about brands or influencers using them for PR or commentary, although there's nothing to stop that. It's kinda against the fundamental ethos: that it's about people not brands. And in that, it enjoys the fact that it's already succeeded, but it inherently doesn't want to "succeed" in the commercial or capitalistic sense. All it needs to do is continue to exist.