*From a business point-of-view (when sideloading becomes more common) the "right...

From a business point-of-view (when sideloading becomes more common) the "right" move for Google would be to fork AOSP, make new Android versions as closed as iOS (while still distributed for free) and prevent any sideloading.

You haven’t been paying attention...

Google has been abandoning AOSP for years and making closed source equivalents.