People here need to realize that 90% of the microbloggers don't give a fuck about decentralization or FOSS. They want something that works and doesn't force them into a ketamine fuled nazi oligarchy delirium. Mastodon doesn't work for normal people. It kind of works if you're a FOSS nerd or some kind of fediverse idealist. (It works for me, because it doesn't drag me into endless flame wars and I'm almost only following FOSS accounts).
My experience with Lemmy is that it is much more functional as in "Reddit replacement". There are of course super few users, but it feels active and engaging (for better or worse). So in theory, maybe it could be a replacement.
But Mastodon has never been a "Twitter replacement". It feels more like a fancy RSS client. Search, feeds and interactions just doesn't work very well.