This misunderstands what happens inside implementations. There ARE low-level bits "down there", but they aren't spec'd. That what the Navigation Controller is: a spec for the low-level bits.

As a part of that process (and Web Components before it), the goal isn't to throw out what has come before (we don't get that luxury out here on successful, non-proprietary platforms), but to re-cast it in the light of a lower-level thing that explains it.

You can go the other way too. Noodle on this to get a sense for it: how much of the <audio> element can be implemented with some JS and the Web Audio spec? Since those connections make sense, why isn't <audio> spec'd in terms of Web Audio, such that you can plug in/out the bits you need when you need something slightly different?