If you can’t delete, rent it out for abuse. Data will become so unreliable it should hurt their advertising business models. Similar to “social cooling”.
I wouldn't mind deleting everything, loading up facebook onto a VPN-facing VM and selling it for cash and watching it burn. Maybe my old account would become so cluttered with garbage, that they'll be forced to finally let me open up a blank new account with nothing on it that I'll park and forget the password to one day. Only then will the illusion of privacy finally be complete.
Citation: I deleted my account, asked friends if my side of old conversations was still there. It is.
I'm not at all sure how data protection laws, such as GDPR, do or should interact with online messaging systems. For instance, if you ask Google to remove all data they have on you on gmail, do they have to reach into the mailboxes of any other gmail users who have received mail from you and delete those messages?
Or do the recipients count as the ones who are storing that data, so if you want your data out of my mailbox you would have to ask me, not Google?
Maybe it depends on jurisdiction.