It should've been a layer of encryption over SMS and remain otherwise stateless and platform agnostic.
Umm what?
SMS has a very short size limit. Implementing RCS as an encryption layer on top of it would require devices to send several messages just to cover a short one-word reply. They also often come out of order so they would need to include a numbering system so the client could piece them back together.
Granted that is already how SMS works on modern devices, but the underlying protocol is woefully inept at modern messaging and completely unviable for what you're proposing.
How should media attachments work? I assume you expect that to just use encryption built on MMS? So media can come through even more compressed than basic MMS? None of the actual benefits of RCS would be possible if it was built on top of the existing ancient standards.