You can interoperate with googles RCS.
If you are willing and able to enter a partnership like Samsung, you can do it fully (including encryption support etc).
Google are determined to not make it easy, and I agree with you, it appears to be yet another messaging land grab.
Trying to put myself in their headspace for a moment, one justification for making it hard is to stop thousands of apps coming out declaring “full RCS support!” through the APIs, then screwing the pooch (through poor security or deliberate back doors or or or).
Right now Google are desperately attempting to make RCS happen, after almost a decade of trying and failing to make various carriers play ball.
They do not want any bad press about how feature poor/insecure/slow/buggy it is right now.