Doesn't have to be Lemmy, they just have to stop using Reddit. If the power users (posters, content creators, mods, etc) really do leave, then the regular users will likely lose interest and leave as well. It doesn't matter if they go to Lemmy, TikTok, or start spending time with their loved ones again.
There are rumors that Reddit will start using (more) bots and AI to generate content, which is certainly not beneath them at this point. The tech equivalent of a lava lamp.