Isn't Google's RCS implementation proprietary anyhow?
Google like promoting the idea that "RCS is public", which it is unless you want to use all the things that people actually like RCS for, at which point it's "Let's talk".
I actually think Google would license its proprietary RCS extensions to Apple for free (it makes Android more compelling if it can actually talk to iPhones, and might reduce some of the "green bubble stigma" (which is bullshit anyway) associated with Androids, but I can't see how it really benefits Apple to do that...
Anyone who cares about the colour of their bubble in text has the choice of buying an iPhone right now. If they don't buy the phone, they don't care enough to do anything other than bitch, if they do buy the phone, Apple gets a win from the purchase. Implementing RCS seems like a net loss for Apple from that perspective.
There is the marginal gain in security/privacy if some people in a texting social group are on Android and the rest are on iPhones. I'm not sure that's enough for Apple to give up the cash-cow though.