Couldn’t say, but I think when Lemmy rolls out account migration features (like Mastodon/Firefish/Misskey etc) we’ll probably see a fairly large amount of people move elsewhere.
Lemmy’s at an odd stage of development right now, kinda federates, kinda doesn’t. As it matures, so will its userbase and I think any potential centralisation issues will take care of themselves. At the moment, users have to physically move (as I have twice already to help decentralisation only to see the place I moved to become very popular lol).