If someone will absolutely not accept talking without disappearing messages, they probably won't go "well iMessage doesn't have the feature so I might as well talk without it", they'd ask that you switch over to an app that does have it.
It’s the same as unsending messages - you can ask people not to do it, but if the option exists, they will do it anyway. I’d rather that options I don’t like don’t exist.
Lost a friend over that, actually. "Well you should have respected my wish to not read those messages" "THEN DON'T SEND THEM"