You're seeing this as a black and white issue. If it had happened to me, the lesson I'd have learned is that Franny and Chester are not trustworthy, not that DMs as a feature are bad
Also, yes there are things which shouldn't be talked in public, be it because it is sensitive and one does not trust the public to know (I for example would not say in public anything I wouldn't want the feds to know) and because saying things in public invites public interference in things which sometimes need to be one on one, which again is only possible through DMs
I think pedi dms and even more so, pleroma chat (misskey does not have that feature) are a good middle-ground between something completely private such as element and something completely public like a post, since it can be audited by every admin and is not "safe", even if not spied on regularly. That is, an admin looking at it for evidence is trivial
