You're overstating the value of the network effect
There are two levels of network effect.
It is a big hill to climb to port your app to a totally different environment. App writers will support iOS and Android, because both are profitable for them. They would be very reluctant to support a new mobile OS, because it would be starting out with zero customers and would entail a lot of work developing to a totally new API.
By contrast, if you already have an Android app at the Google Play store, if there was a 'alternate' flavor of Android using the AOSP that was either supported by an open source project or run by a big company like Microsoft, it would be a tiny hill to climb for those devs to port their "Google Android" app to "Microsoft Android" or "GNU Android". Same API, they'd just have to test that it works with the alternate location services etc. and upload to a different app store. Nothing Google can do about that, any more than they could prevent an Android dev from porting his app to iOS.