actually the only way for them not be paraller is to drop dx12 not just async
and also since most of the graphics engines out there(the popular ones)have already native dx12/vulkan support i dont see why they would drop it its not in their interest to do it unless their pr team say so
Every game engine out there has CPU multithread support for many years now but still a lot of games have a low number of threads,just because it is there does not mean that it is used automatically and that they(devs) will have to drop it for it not to be in the game.
Devs will have to put a lot of work into their games to make them parallel,was(is) true for CPU will still be true for GPU/dx12.
Dx12 allows the driver thread,or the work the driver thread would do,to be split up into smaller parts,instead of having a driver thread with x% cpu usage you can have a number of threads with x/n % cpu usage.
Example 30% in one thread or 5% with 6 threads.
This changes nothing for anything in the game engine that is not graphics based.