I'm assuming (for now) that discrete GPUs will still be featured in higher-end machines, so the Radeon drivers will appear over the next couple of years.

I think Apple is done supporting other people’s GPUs. Their graphics are pretty good, they can match NVidea and AMD with TSMC’s help if they want to.

There is still PCIe bus and Thunderbolt, technically it is still possible to connect NVIDIA/AMD GPU to Apple Silicon, but we need enough bus (x8 or x16) and drivers.