The problem is that it's not really up to Fairphone. Qualcomm and Google have to collaborate to provide the artifacts that Fairphone packages and signs for their devices. If for any reason they're unable or unwilling to do that there's nothing Fairphone can do. (and they have pretty consistently failed to do this after just a couple years. In the past it sounds like Fairphone has managed to hack around it with varying degrees of success.)
This is why using SOCs with poor support and closed drivers like this is a terrible idea.
It is for all the phones sold in EU. So Qualcomm has to provide them for all other providers too. They might release updates for Fairphone SoC as well.
I agree that the platform should be open. I played with PostmarketOS on one of my old devices and I really wish I could just install Linux on my other devices to make it really usable for the years to come.
This is why using SOCs with poor support and closed drivers like this is a terrible idea.