The problem, as a few others have noted, is that RCS as google wants Apple to use it isn't a standard based successor to SMS, it's some standard with a whole bunch of google bolt-ons and their own servers in the mix because of inconsistent carrier support. The carriers are honestly the biggest problem here - in the US, where this problem is most prevalent because other services are dominant in most of the rest of the world, they've dragged their feet on any standard consistent replacements or updates to SMS, in large part because of the desire to MiTM message traffic and the data mining it provides, as well as load on their own servers.
If RCS as google wants it used was truly a rolled out fully supported standard I don't think Apple would take such a hard line.