I think its largely a showcase of an ideal: it changes the entirety of the interaction model, and this is ideally what the extension system should be capable of

I think its largely a showcase of an ideal: it changes the entirety of the interaction model, and this is ideally what the extension system should be capable of, even if relatively few people want such a significant change. And, ideally, we should see extensions upturning the interaction model as users experiment with alternatives; that this is no longer possible, that the fundamental model is defined and hardcoded (with minor flavour differences), is, I think, a significant loss. Its a shame there are few extensions like vimperator; its a greater shame there are none.