That "F" (as in freedom) is certainly eroding. Perhaps not by its source availability directly (although without any drivers, what is the use?), but very much by a company trying to lock you out of all the goodies that once came with it.

Even if Google would stop open sourcing AOSP, I think it would be much easier to fork AOSP than to start a new Linux-based FOSS mobile operating system from scratch

Obviously even maintaining AOSP yourself requires a huge effort and a lot of people would need to donate development time/money.