I'd say the reason generic kernels/images are missing in the ARM world is because people generally don't run their own software on the ARM hardware they own. And the reason for that in turn is that often people don't have the ability to run their own software on their ARM hardware. Which brings us back to the GPL - this is standard "Tivoization", something that the GPLv3 tried to prevent, but unfortunately Linux is GPLv2... Still, for drivers that are open source and upstream, a practical consequence is typically that people are allowed to run their own kernels on the associated devices - which I think would give the PnP, generic image world you want.

This is all just to say, I don't think "GPL drivers" and "PnP generic images" are as separate as you're suggesting.