What you call "ossification" others would call "stabilization" and "standardization". Not to say that the inertia of (some) kernels is not excessive sometimes but I'm not a fan of the "fuck it, we'll do it in userland" attitude. That creates big, opaque application blobs that nobody understands and knows how to debug with standard tools.

I feel like Google in particular loves to do that stuff because while they control the internet and the browser, they don't (yet) control the OS so they have a very strong incentive to push stuff up in order to retain control. They don't have to ask anybody to add things into Chrome, they have to get third parties on board to extend the kernels (outside of Android at least).