It will be different, for sure, but big YouTube personalities who make most of their money from brand deals already could actually see themselves come out ahead in the fictitious world where everyone leaves Twitter and Reddit for the Fediverse.
Hosting videos is expensive, but the big names make more money for YouTube than covers said names' usage costs, and if people ever had reason to realize that they can follow PeerTube from Lemmy/kbin/Mastodon/Calckey/etc., then Bob's your uncle.
Of course, we don't live in that world. But it's a nice thought.