If Apple does adopt RCS, it's likely only as a fallback for iMessage, much like SMS today. And rather than Apple abandoning control over their messaging platform, I'd sooner bet on Apple releasing iMessage and Facetime for Android with a small subscription fee. Probably in the range of US$1-$2/month.

If there's one thing that Apple hates more than lost profits, it's handing over control of anything to carriers. Cheap-subscription iMessage/Facetime for Android would strangle RCS/Duo in the cradle and put a thumb in the eye of the carriers, while bringing in some steady and significant regular revenue. Let's not forget that Apple is already offering a subscription service on Android, Apple Music. They're not averse to the concept.