I think one limiting factor is the fact that most posts aren't particularly deep or insightful, so the intellectual space from which one can engage on the subje

I think one limiting factor is the fact that most posts aren't particularly deep or insightful, so the intellectual space from which one can engage on the subject is pretty limited. So most people who engage with content of this sort are just clout chasers trying to get attention or scammers. Is it really surprising that if Elon amplifies some post with a single exclamation mark all of the replies will be completely inane and full of spam? Another factor is account size. You can still find interesting replies in accounts that are relatively small, especially when the author takes time to manually hide spam and scams. But once an account goes beyond a certain size they just become a magnet for nonsense. It's pure quantity over quality. Elon has the biggest number of followers but the quality is abysmal.