I doubt it would ever get to a full replacement, but that’s fine. It doesn’t have to become the new Facebook to still be competent and successful in it own right.

By their nature, both Lemmy and Mastodon will be unlikely to have the same kind of reach, simply because they have the added complication of Federation and all of that on top of everything, which complicates things a bunch.

You also have some decent competitors starting up, which would also split things. Lemmy is competing with Reddit, and the similar services, like Kbin/Threads/Tildes.

Sure, the competition is a bit less direct for sites that Lemmy Federates with, but it’s still going to split the user base in some way.

Lemmy also has a few technical hiccoughs and other issues that get in the way of it really becoming popular as it is now. Lemmy.world, pretty much the biggest instance around, suffers from several issues where the code-base simply wasn’t designed for the sheer volume of users and activity that they’re seeing.

Lemmy’s devs would have to go and fix those problems before it could sustainably become a mainstream app.