It's still a lot more work than you might think, and updating the backend stuff is more difficult and thankless than building a shiny frontend.

Telepathy does a similar thing to what you're describing (using a core framework with components for different transports) which is great in principle, but look at how long it has taken/is taking to drag upstream kicking and screaming towards implementing some of the more vital XEPs (XMPP extensions) for XMPP on mobile.

"someone can just do X" =/= "someone will do X"

Edit: also, see how often something breaks in Hangish, despite the developer's continued input. It's a great client, but each change to Hangouts can break everything without warning, and take a long time to resolve. In the meantime, you're stuck with a broken client. For a "translation" library to work you need quite a lot of notice in advance about changes to protocols, which won't happen with proprietary messaging apps. Ironically, Hangouts is based on XMPP with some proprietary extensions thrown in...