The only way for carriers to interoperate with each other is by having an intermediary party which limits their freedom of customizing any given specification.

The only way for carriers to interoperate with each other is by having an intermediary party which limits their freedom of customizing any given specification. This is applicable ESPECIALLY to large-scale carriers of US, which are big enough to customize every spec and force their suppliers to adopt it. The only reason why Google's RCS works better than any attempt before that, is that Google is forced to unify the implementation. It's not their spec, they are part of the RCS working-group like everyone else, but by stepping in they instantly reached a cross-carrier rollout-scale where they can't comply to every custom requirement of ATT, Verizon, Vodafone, T-Mobile unless they all agree to make it part of the standard.