"Anyone can implement a client" isn't the strongest argument when that's also true for most chat services as well. It's nice that the protocol is out in the open but it's effectively a service provided by cell providers and locked to them.

RCS could have been amazing as a general-purpose protocol for short datagrams that could be sent/received in areas where you can't get a data connection. It would effectively enable every chat app to work when the network is under load but carriers opted for the walled garden.