Which is why you don’t see successful messaging companies rushing to open their systems.
What company wants to spend the effort to build a successful messaging platform, only to open it and see some nobody come in with feature parity _and_ the ability to reach/convert every single one of their customers?
Open protocols would have worked great if that’s what was mandated _before_ these established products were in the market. Now, it’s going to be an uphill fight all the way, and companies are going to do the absolute bare minimum—-not just what they can get away with technically, but what they can get away with legally (e.g., Apple adopting RCS so they can point to RCS as the common messaging denominator, thus avoiding opening up iMessage itself).