So much for an "open protocol". It may be open on paper but if Google controls it in this way it's not open in practice.
Luckily it doesn't really matter here in Europe. Nobody uses SMS and by extension, RCS, for personal communications here. I don't have a rooted phone and I don't even have RCS enabled and I ignore all SMS (notifications blocked). And I don't use a Google account on my android phone.
I just don't want to get roped into yet another communication method. I use WhatsApp and Telegram.
I'm not really happy with those either as those are walled gardens too, and collect too much data on my phone. I'd love for something truly open like matrix to gain adoption but I doubt it'll ever happen. So I just run my own matrix server with bridges.
But clearly RCS is another protocol controlled by the vendor and as such no better than WhatsApp, iMessage and all the others.
Apple does not punish users for jailbreaking or sideloading to their devices. There is no loss of function ‘due to security reasons.’