SMS/MMS needs to die at this point. I am glad to see Signal take a hard line here even though it will cause some headache for users, of which I am one, though I do not use it to send outgoing SMS/MMS.
These protocols are insecure, not private, and fundamentally incompatible with Signal's mission. Supporting them at all, while highly convenient, is a queer oxymoron for an app like Signal. We have to rip the bandaid off eventually.