I don't think anyone can tell where we're going. Mastodon.social was the largest instance by far for some time, then at the deluge, it splintered.
Part of the reason for Mastodon to fracture is specialization - each instance does something unique. Maybe Lemmy will do the same, maybe not.
But if we end up with 3 primary instances, it's still decentralized - I think the most useful feature of Lemmy isn't that we're spread out, it's that we could be.