RCS the oficial standard is open yes. But Goggle's implementation has added proprietary bits and changes that aren't in the spec. Even if they opened their servers nobody could use them anyway.
That is correct. However, Google does control Android and has not added the ability for other applications to interact with RCS, other than Google Messages and the Samsung messenger.
I think the holdup is that previously SMS was paid for through your carrier, so Google can let any app use it and no one loses money. Now the main RCS instance is hosted by Google, and I guess Google doesn't want other apps freeloading on their servers since you're not paying for RCS directly like you are paying your carrier. It really sucks and I hope it gets resolved soon, especially with Apple joining in. We also need a good open source RCS server and client.
Well in that case it cannot replace SMS entirely until it's completely opened up so that absolutely anybody can use it. Either that or the more terrible alternative is that they are going to not open it up so that other apps can use it and literally kill every other SMS messenger in existence. Except for Google Messages and whatever Samsung uses