What you're asking for is awesome, but.... it's really hard to do well. The extra abstraction is hard to get right. Instead a bunch of decoupled components playing well together, you can wind up with a bunch of chaotic components having their dependencies changed out from under them, and seriously raising the barrier to entry for newbie developers who need to figure out how to put humpty dumpty back together again to actually build an app.
Rails 2->3 tried to the general route you're talking about (but still with language API connections, not REST. That would be even harder in some ways) --- with, well, mixed success. And Rails starts out lower level (much less UI built-ins) than django, another thing that would make it even harder.
I've had many failures in personal homegrown projects. :)
Abstraction is hard. To do super high level of architecture abstraction _well_ takes experience and skill, as well as more developer resources. It's easy to go off into architecture astronaut adventures, and you end up spending all your development time trying to keep the parts working together, significantly sapping resources to actually, you know, provide new functionality.
That said, there is certainly such a thing also as too little abstraction and too much tight-coupling too, obviously. There is a happy medium.