In theory it would be trivial to open up the big networks, if they were each willing to expose a public, open API. The APIs don't even have to be interoperable directly, they could let the client apps deal with that. It could be rolled out super fast if they wanted to – couple of months.

But of course none of them actually wants this, so I expect they will fight it tooth and nail, while not appearing to do so. Meaning they'll drag this out for as long as possible while blaming each other. I expect RCS will be a perfect red herring for this, because of its complexity and the ability to blame interop issues on each other.