I use the term "proprietary" in sense that it's their own unique protocol that nobody else uses. (Don't tell me about their interop with CM, its partnership, not federation.) Or you know some alternative compatible SMS apps that use libaxolotl or Axolotl protocol? I don't. Would love to hear there are some.
Fair enough, they do "own" it, and can change it at their own discretion, and that is always bad. However, that's how many standards come to life. See SPDY and HTTP/2.0 and probably (no facts to back this one) most of the XMPP XEPs.
Not sure about the SMS side of things (which is being dropped anyway) and how things work there, but using the phone number as an identifier makes federation without a central authority infeasible. I've been wishing since day one that they'd allow xmpp style usernames too in order to make federation possible.
