They are both valid threat models, but ones which for me have different meanings.

Screenshotting or photographing the screen of a device owned my my intended message recipient is a reasonably small problem to me. If my recipient wants to expose a message I've sent them, they're going to be able to do that. I never expected any more privacy for that message than I'd have accepted based on my trust in that person.

MITM servers are a whole other thing. Large scale surveillance of all users of a specific server, "full take" collection and searchable databases of messages available effectively forever to unknown current and future opponents?

Different threats. Yeah, I'm happy enough to accept the risk of cameras in the hands of my correspondents. Way happier than I'd be with MITMable servers (or services that can add "ghost users" as the UK seems to be proposing).

If you have to get into a legal battle with someone about misuse of information, it's much better that you are able to focus on the sender and the recipient of the information as potential sources for that information instead of also having to go after every potential network hop as well.