Even when Reddit exploded in popularity there were still plenty of great, but smaller, communities. I really enjoyed being part of Reddit until the API changes when all the decent moderators quit. The quality of Reddit went off a cliff shortly after that happened.
So I think Lemmy will be fine even if it explodes in popularity. Capitalist greed that betrayed the OG members and mods who built Reddit is what is making Reddit a cess pool, not the number of users.