I guess if Google closes down AOSP it would get forked, and the fork would probably be a separate thing from the current Android distributions. So that the landscape would continue to look a bit like today, except that AOSP would be an independent thing.

Then I guess it's possible that Google would seek to make android apps incompatible, gradually making the whole thing kinda pointless. I can't say I'm using Android for the great UX - I'm using it because it supports a few apps I continue to be forced to use. If I can't use them an Android any more I'm switching to Ubuntu Touch or PostmarketOS in a heartbeat.