It's weird to me that so many people don't seem to get that. SMS/MMS are terrible in many ways, but they have the benefit of the universal network effect, without even trying.
Sure, if I want to send someone videos, or do regular group chats, I'm going to find an alternative platform. But if it's just a casual contact, or I don't anticipate needing these things, I'll just stick with SMS.
I think this is exclusive to Jibe and Google Messages but a widespread (in terms of installs) proof-of-concept is still pretty cool, and better than nothing.
The bigger problem is adding E2E to SMS is going to attract the FBI's ire, they really really really like reading everybody's texts.