While WDDM has some backward compatibility
defined for it, there's nothing to say ATI/NVidia have to support
XDDM forever on new designs. They could drop it at any time,
simultaneous with stopping WinXP driver support.

If they want to abandon the population of XP users, then
used hardware and support archives would take up the slack.

Durable goods like computers should not be obsoleted and
scrapped/landfilled because they're 3 years old, or because
a big OS vendor wants to DRIVE obsolescence of hardware
without regard to speed benchmarks.