I guess I was thinking of cruft differently. All the messaging and bots are reducing UIs back down to CLIs again. Which is pretty much the sparest UI there is. Except perhaps for voice, the other UI gaining traction on mobile. So, yes, the physical size of mobile devices is reducing practically all the cruft.
- The phone OS has a shell, usually written in native code.
- The messenger app runs on top of the shell, usually written on top of some type of VM.
- The messenger app contains a web browser that renders other apps, written in js.