Earlier quoted context omitted.
I don't think it's this simple. RCS is implemented by carriers and is a feature of mobile service like SMS. It has a qualification/accreditation process. iMessage is an internet service. If I were Apple, I would not budge. I haven't used iMessage, so I don't know how it behaves, but my own personal RCS experience has not been good. I am on Google Fi and make heavy use of Google Messages and specifically their web app…
Respectfully, this comment solidifies why iMessage is superior. Try sending a 4K HDR video, or low-quality video, files, or photos with a resolution greater than a Nokia phone from 10 years ago, or a multitude of other things over SMS. Signal, WhatsApp, etc. don't come close to what iMessage offers seamlessly. For iPhone/macOS/iPadOS users, SMS is garbage. Yes, it's a closed system; but it works without a cell signal…
It's not useless, but vastly better for short text conversations than anything else. Not surprising, it's SMS.