Oh, reddit's still going to be around for a long time, and I don't think reddit clones were and are any real threat to reddit (See Voat, or any of the crypto based reddit clones). However, Lemmy is different in that federation is a revolutionary change to the reddit format just as nested comments on reddit is a revolutionary change to traditional internet forums.

So, a likely scenario is that high effort content creators are going away first, leaving the average user who only notice the content getting worse and worse until they leave too, and the dreg will get more and more concentrated as more regular people leave, which lead to worse content, turning it into a death spiral.