The lessons from Twitter are clear: most people will not leave bad social media without a better replacement. They're attached to their history, and attached to their routine. It getting worse does not change that.
Mastodon didn't scratch the Twitter users' itches, and it doesn't look like Lemmy will scratch Reddit users'. These aren[t the people that populated the Internet 20 years ago.
They're the people who never would have touched it, because it was too technical, had too high a barrier of entry, and saw it as niche.
It's time to stop focusing on whether Reddit succeeds or fails. They're not going to fail. Instead, it's time to make an internet of niches again, for ourselves, without the Twitter and Reddit users.