Dumb question:
What's to stop nvidia fixing their DX12 drivers to keep up?
Unlike overtessallation that can be reduced, you cannot fix something that's fundamentally absent in the hardware. Card A can do tasks X+Y faster than card B can do X, then Y.
Another way to look at it is with parallel Async Compute you can do something 10-30% faster than using a serial method of waiting for A to do B then waiting for B to do C. That's why Async compute is not at all like tessellation. Overtessallation reduces performance on all hardware. Excessive compute improves performance on architectures that can do it, and penalizes architectures that cannot since they can't take advantage of parallel computation. Async compute is like a person having 3 arms (AMD) vs. one with 2 arms (NV). You give them 3 jobs to do. The 3rd arm is only allowed to perform specific tasks, but if you don't have it, you will never speed up the work since neither of your 2 arms can handle the specific tasks of a 3rd arm at the same time while the 2 arms are busy -- that's NV right now in DX12.
In simplest terms, Async Compute is like multi-cores for CPUs - X cores run audio, Y cores run AI, Z cores run graphics at the same time. NV can't split compute and graphics workloads without a huge performance penalty, making it a worthless feature. It basically means NV can do compute work loads really well (Tesla) and graphics workloads really well (GeForce), but it cannot do both concurrently well -- AMD's GCN can under DX12 or OpenCL.
All that extra power consumption from is partly because GCN has so many more functional units, more advanced command processor, ACE engines, more shaders, etc.
Having said that, some of these DX12 games don't even appear to use much Async Compute, which means non-GCN architectures suffer since the developers probably don't want to spend extra $ optimizing beyond console architecture. This has been called the 'console effect'. I believe NV tried to combat this with GameWorks but they don't have unlimited funds to buy out every AAA game.