RCS isn't as open as SMS, it's just as proprietary as iMessage, Google has just expressed a willingness to let other companies use it. They're playing nice because they're the underdog in the US market. If RCS becomes the new standard, Google will exploit that fact.
Not quite true, RCS (formerly Joyn) was designed by the consortium setting the cellular network standards as replacement for SMS and MMS. It was mainly expected to have the services run at the carrier network - though having it run somewhere else was an option (just like with SMS). Which is what google is doing now, they're running their own servers.
Efforts to develop the protocol started almost two decades ago, and it was supposed to be taken into use about 15 years ago - but the protocol is just horrible, so the main surprise is that it didn't fully die, but google ended up implementing it.
They could've done pretty much anything at that point - implementations for this were non-existent, and nobody really wanted to be the first to implement it, as - like I said - the whole thing is just horrible. It was requested back then when we started work on the Jolla1, but after having a closer look at the specs we laughed, and it was never brought up again.