Google tried the same thing Apple did long before RCS when it made Hangouts the default SMS app for Android. Conversations could be upgraded from SMS to Google's internet-based chat protocol if the other person had an account; it was even available for iPhones, but it couldn't be an SMS client.
Lets be honest, Hangouts (which of the three versions) was a crappy chat app that they wanted to boost the usage of. It wasn't intended to be a functional SMS replacement.
Carriers didn't like it and Google caved.