Users are encouraged to rotate their PGP keys in the same way that LARPers are encouraged to sharpen their play swords: not only does nobody do it, but the whole system would probably fall apart if everyone did.
If messages can be sent in plaintext, they will be sent in plaintext.
The clearest example of this problem is something every user of encrypted email has seen: the inevitable unencrypted reply. In any group of people exchanging encrypted emails, someone will eventually manage to reply in plaintext, usually with a quoted copy of the entire chain of email attached. This is tolerated, because most people who encrypt emails are LARPing. But in the real world, it’s an irrevocable disaster.