RCS itself is a protocol, and it's defined and developed by GSMA, not Google. Googles biggest involvement is setting up their own RCS network but that wasn't even necessarily required. It was done in response to inconsistent implementations among other carriers, and Google essentially said "if nobody is going to do something about it, we will".

Android was not the only OS to adopt it. MS adopted it as well. Though they didn't get much sticking power, WP10 had white a few decent models running it.

If Samsung's default messaging app sent high quality images and photos, but only to Samsung phones, I'd be equally pissed.

Really the only reason Apple is going to support RCS is because their feared EU regulations coming up to force their hand. They are the only smartphone OEM to not support it.