And now your messenger app needs a launcher, switcher, notification manager, search, per-app settings pages, etc. And maybe its own process model/scheduler, to prevent rogue apps from slowing down the messenger app. And apps within the messenger app now need to be cross-messenger-platform on top of being cross-webview-platform.
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.