Are you making any sort of posts in those "mostly dead" communities? There might be more people looking to join but had the same thought as you. Even a quick hello post could help a lot.

Lemmy is more picture oriented, like Instagram sort of. Reddit at least I'd you used old and shut off custom css, was more textual. -1 for Lemmy there I'm afraid.

What makes you think Lemmy would die that quick?

Lemmy existed years before now. It will exist for many more.