Well, i bought the Fairphone 2 because i searched a newer replacement for my Jolla Device.
For me it doesn't matter if its AOSP or CM Base - For me its importand that SailfishOS runs good.
As a Computer and Linux geek i also often flash CM on the Android phones of my friends and customers.
Because the stock ROMs sucks. They are bloated with garbage software, slow and sometime impossible to root.
Fairphone Open brings a very good AOSP ROM which is not bloated and which is rooted by default.
Yes, there are lot of software problems, but i doubt, that the CM ROM would be more stabel than Fairphone Open. I think it will be as good as Fairphone Open, when it comes out.
Lot of Problems are Modem Firmware Based - and here we need to use binary blobs. Also CM will use the same binary blobs like Fairphone Open, and bring the same problems.
But that is all speculation. Lets see and compare, when the CM or LineageOS is coming out for the FP.
But i think, it will not coming out....
So dear Fairphone Developers: Please fix the most important bugs like the battery-drain-bug
And if the base is table and good enough - the communtiy will make the other OS avaiabale:
SailfishOS, UbuntuTouch, CianogenMod etc....