They cannot roll it back. Anybody can create a hard fork, and nobody is forced to use that version. In fact, the version that was not rolled back still exists, and we are all free to use it if we wish!
This ignores network effects at play. As soon as large parties or developers of popular front-end software announce their support, the majority of smaller users will quickly follow - simply because not following would put them on a massive disadvantage.
The developers of the clients do have a massive advantage here.
I disagree, a fork with a non-Google app store could absolutely threaten Google.
The Oculus Quest runs an Android fork for example. Are you saying if Google gets into VR Facebook will not be able to compete because of the fact they are using Android?
The developers of the clients do have a massive advantage here.