It can't last. Right now, lemmy/ActivityPub is in the "early adopter" stage of the tech hype cycle. The only people here now are the people who are willing to try out something new. If there are enough "early adopters", Lemmy will become interesting, and then the normal people will follow. This would lead to an "eternal September" effect of declining quality. Then they're followed by the spammers and people looking to make a profit.
If basically feels like reinventing Usenet, with maybe some extra modern features.
There's one big weakness. There appears to be some sort of shared blocklist. If people wind up being placed on the list for petty reasons rather than genuine misbehavior, that could become a problem. I.e., the people maintaining the blocklist decide they disagree with X politically, and then X winds up on the blocklist even though they really weren't abusive. Then people running nodes are going to have to start manually reviewing the blocklist and making exceptions, which most people won't bother doing.