RCS is not really an open standard though. If you want encrypted RCS with android phones, you can't unless Google lets you. At least that was true last I checked. I'm guessing it hasn't changed.
Or more specifically: it's a different product ("Google Messages") that just happens to be based on RCS.
They do have some partnerships with hardware manufacturers that ship Play on their devices, and they will preload Google Messages in there as well.
In essence, it doesn't exist in AOSP, and doesn't really live side-by-side with a normal messaging app (i.e. one that only does baseband native messaging), I wouldn't be surprised if the partnerships and preloading conditions state the manufacturer can't ship their own version (I think at least Samsung had to drop their own "Samsung Messages" app as reported in one of the reviews of a foldable display phone).
In a way, RCS made no difference, and whatever Google did was mostly just to compete with Meta (both FB Messenger and WhatsApp). Fun fact: Google Messages is closer to Matrix than it is to iMessage in terms of comparable technical features.
Thankfully we have Signal, which solves the problem better than either platform option.
I have almost no way to convince anyone other than people very close to me to use it due to the (lack) of network effect. If they could just use it instead of the default messenger then it's a dramatically easier sell.
Obviously it's up to the Signal Foundation about the direction they take but I don't know if I've seen anyone agree with the justifications.
Google and Apple wrap up their locked down BS with SMS for the same reason. It's by default free of network effect but passively pulls people in.