As of today, there are no phones running Fuchsia, so it's hard to evaluate if it's going to be useful for any kind of control. Does it have feature parity with Linux?
Forking can be practical without competing for the same use case.
Both of these are true, and it is indeed possible that Fuschia is intended to be genuinely open.
However that seek unlikely given that there are many actors who would like to exclude Google services from their devices and Google is at the same existential risk if that happens as they were when they introduced Android.
And yes forking can be practical, but it very often isn’t, partially in the case of a very large corporately sponsored codebase. So ‘we can just fork it’ doesn’t mitigate the concerns.
As for forking, that option only becomes practical if the community version can muster adequate resources versus the Google controlled version.