cubefox parent
You are misunderstanding the function of RCS as making different message services compatible. iMessage doesn't exist on Android, hence the necessity of an open protocol.
The problem is that RCS has completely failed at making even different RCS servers compatible.
Carriers like AT&T and Vodafone that were early to adopt RCS never even managed to interoperate with other RCS carriers - you could only send RCS to other AT&T subscribers, or other Vodafone subscribers. Then they all gave up and just adopted Google's servers. So now RCS just means "Google Jibe".
It's a failed standard. It never worked.