Your messages going to and being handled by other services means you'd be subject to their TOS and privacy policy as well.

This is true of literally every one of your contacts, too. When you send someone a message, they can screenshot, copy, archive, and forward however they see fit (and most people don't govern themselves by any kind of TOS or privacy policy). Which then means that if any one of your contacts chooses to use another service as a bridge, or as an archival tool, you're naturally going to expose your messages to that service, on that contact's terms.

But that isn't about interoperability per se. It's about how other people store and use their copy of data shared between multiple users. Apple iMessage isn't interoperable with anything, but users still have conversations archived all the way back to the beginning of the service over a decade ago, and can choose to export those messages to be saved elsewhere. (For example, I use a bridge for iMessage so that I can view them on my Android phone, but the mechanism is software that leverages the Mac's accessibility API).

Some of us are data hoarders. If you're gonna have a conversation with people like me, you'll have to trust that we don't use those archives in a way that either inadvertently/negligently or intentionally exposes that data to some bad actor. I'd like to think I do a good job of respecting my friends' privacy, and secure my systems, but I'm probably not perfect.