Here are more:
- (jumping off of your second point...) Play Services does more than just handle stuff you sign into as a user -- it's also a dependency for everything from push notifications to screen casting. This actively poses issues building competing platforms, in that in order to give developers a path to shipping in your ecosystem you have to provide functioning alternatives to all of those ancillary features. The compatibility issue also impacts user adoption, and then the user adoption and the barren marketplace impact each other... Even the combined resources of Amazon and Microsoft weren't enough to overcome this. (Facebook did, but I'm also not sure forking the OS into a separate VR platform is necessarily the same thing.)
- It also comes with integrity checking, so even if you do find a good third party image, and sideload Google packages, numerous things won't work unless you take part in a dumb arms race that ironically requires you to also root your device. By which I mean a feature that was originally built for banking applications is now used everywhere from streaming services (as an additional layer of DRM) to gacha games (for anti-cheat). This is actually the entire reason I dropped Pokemon Go, personally.
Obligatory link to the excellent Ars piece on this topic: https://arstechnica.com/gadgets/2018/07/googles-iron-grip-on...