only for a relatively short period of time.

exactly the same case is with posts about news.

I think using blockchains to hold encrypted data is sort of antisocial.

Correct. So conclusion should be... that others should benefit somehow for storing those messages for you for eternity. My project prio recognized that... that is why we want to introduce private paid priority messaging, and at the same time our public messages will be always free, because those messages contributes to common knowledge.