Even today, inter-network SMS gateways are tied together with damp string, so I'm not sure the carriers are the obvious people to implement anything more complicated.
Moreover, if RCS is built on SIP. the only reason I can think of for not running it directly on top of IP is the inherently ougoing-only connection model of the current NAT'ed IPv4 implementations run by the telcos. I assume IPv6 would solve that problem better than some bodged-up system for generating carrier income.
I think that's the first time I've said something favourable about IPv6. I need to lie down now...