Google is the only one that allows “End to end” encryption.

Allowing and implementing are not the same things. They implemented encryption in their RCS services. They don't allow everyone to use their service, but they built and own it so that's their right, I guess.

And practically speaking google controls the standard, they have over 800 million users out of the total possible 1.2 billion.

Can you elaborate here? How do they control the standard? Specifically, I'm not asking about their implementation of RCS, because of course they control that, but their implementation is not the same thing as the standard itself.

It might not be a monopolistic standard in theory but it is in practice

It's widely understood that it's difficult to implement a competent web browser. That's why there are only a handful of browser choices. This doesn't make HTTP a monopolistic protocol.

Saying the RCS standard is a monopolistic standard makes zero sense to me, even in practice. We are quite literally discussing another vendor entering the market. If you run a telecom and want to implement RCS, you are able to do so. If you are a phone manufacturer you are free to implement RCS in your software stack. None of this is easy, but it's possible and so this isn't a monopoly situation as far as I understand it. Google wanted to compete with iMessage so they built a competitor on a proprietary but open global standard, the standard which is meant to replace SMS and MMS messaging.