And if other instance owners have access to the private messages of people on every instance, that is a shockingly large flaw. I’m not exactly sure how insecure private messaging would be here. Not that I have people to message. But it being centralized would be more secure if decentralization would allow a much larger number of people to have access to something that, really, should be private.
There are an overwhelming number of people I don’t think are savvy or cynical enough, call it what you will, to understand that just because they call something a private message - or just because it’s supposed to be a one to one interaction - doesn’t mean no one else can see it. I would think, if anything, an overwhelming majority of people who send a private message/DM on a social media assume that no one else at ALL has access to that information.