Flatpack is only a piece of the puzzle. I remember in early Android version, an app could increas gyroscope query frequency (i.e. a racing game demanding precise phone tilt), then crash and the gyroscope would drain battery within hours. And again — this is only one example.

The ecosystem must grow — to this day, I cannot set Immich as my default gallery app on LineageOS. So I take a photo, and can't immediatelly look at it. And Android is already mature. There must be a standard and secure way of exchanging calendar events, notes, photos. Developers must adopt this new ecosystem — it takes years.

The best option we have right now is to pressure Google to allow alternative to Play Services and also sponsor AOSP development outside of Google. There are numerous Linux distros, including commercial ones, I don't see why we can't have numerous Android flavors.

I made a full comment in this thread. The bottomline is

1. Sandboxing of resources both hardware (gyroscope, network devices) and data (photos, music) takes a lot of trial and error.
2. There is a need for an ecosystem: i.e. apps sould be able to create calendar events, or access shared mediaplayer — also with permissions
3. Developers need to adapt to the software ecosystem
4. Hardware companies e.g. smart watch, projectors, TV need to adapt

It all takes years.

Linux phones are around for enthusiasts since Nokia N900 (which was/is a masterpiece) — yet nothing is remotely close to a mainstream phone.