I don't understand the issue. If Fairphone wants GOS can't they just compile it themselves with whatever unsupported hardware based security features disabled? Ditto for any given end user. It's not as though we're talking about a device with hardware specs locked behind an NDA and drivers that only support outdated kernel versions.
Forking GOS and porting it to a new device is a huge and unreasonable ask for an individual user. Even less so when you include the fact that you'll need to keep it up to date.
I understand that bringing AOSP to an unsupported device is a huge endeavor. But fairphone already has the driver situation sorted out. What's the complexity here? (Genuine question, I'd like to correct any misconceptions about the android ecosystem that I have.)