Earlier quoted context omitted.
There are a handful of devices that can run mainline kernels via postmarketOS: https://wiki.postmarketos.org/wiki/Devices That being said, the project is still early, almost nothing supports cellular data, SMS, or phone calls.
Yes, thing is, the amount of work that seems to be needed to bring up a mainline kernel "properly" on even one of the most open and best-documented SOC's in industry (the one Purism chose for this phone) is already a bit scary as shown by the OP post. Now imagine multiplying that by the zillions of phone models that Cyanogenmod used to support or that LineageOS still supports today. That's basically the scope of work…
More and more the most important question I have when I consider to buy a device is whether the mainline kernel is supported and can be installed. If not, I move on.