I don’t think Lemmy will reach or overtake Reddit. That’s a good thing in my opinion, because massive platforms come with massive moderation problems that aren’t so easy to tackle for decentralised networks. We’ve seen that when someone posted kiddie porn and several servers went down to scrub the filth from their systems.
If anything, Lemmy already has a pretty high amount of troll communities, thankfully mostly contained within their own servers, which enables separation through defederation (speaking of defederation, I’d love to have an option to block servers on the user level).