Honestly, SMS/MMS isn’t THAT bad for that kind of communication, apart from the lack of encryption (and, if encryption is necessary, SMS is obviously the wrong tool for it) and persistence (my employer bought us all work phones so they could save conversations for regulatory purposes, seems it was cheaper than the potential regulatory fines that could occur for conducting business over non-persisted channels). If SMS/MMS truly were that awful, I’d probably be a WhatsApp user and wouldn’t bother with the Messages app. So, while a secure, modern, open standard would be great, honestly SMS/MMS is probably good enough for most people.
Seriously? Try sending a picture or video on SMS/MMS and see how well that looks. What about all the other features, video chat/facetime, stickers, tapbacks, etc. SMS/MMS is so antiquated it should only be used as maybe an emergency fallback, if that.