I think the people that value being on a decentralized service will stay on a decentralized server. The people that would abandon one platform to follow their favorite "high follower" poster are normies that never cared about what service they were using to begin with. Meta may absolutely take a large share of users to their platform in the future if they shut off federation and our favorite celebrities and shitposters are no longer visible. But I don't really see how that is any different than Twitter currently having all the celebrities and high volume shitposters. We already can't see them. The EEE argument just strikes me as sour grapes that "their" users are going somewhere else. And I'm on the fediverse (both Mastodon and kbin) so I see the value here. But I'm not going to get angry that normies don't want to put the effort into learning this ecosystem when they have their own lives and struggles and a limited number of social causes to care about.
Now what does bother me is Meta having an outsized influence on the development of the protocol of ActivityPub. We've seen something similar to this with Google using Chrome to push some additions to how browsers handle HTML standards/elements, like supporting DRM.