Again you are focusing on only the bonus. The fact is, the 390 is cheaper and performs better across the board. Period. Ignore the ASync Compute part, that's only a bonus. The 390 is just the better all around choice, excluding any thoughts of DX12. More memory helps with higher resolutions and FPS has always been better at higher resolutions, now with the Crimson drivers it performs better at 1080p too.
Look here:
http://www.guru3d.com/articles_pages...review,24.html
http://www.guru3d.com/articles_pages...review,25.html
The 390 is a better card than the 970. So the 970 is a bad choice to begin with. You add in the -bonus- that it supports DX12 better through ASync Compute which is arguably the most important feature of DX12 and the 970 looks even worse. But even IGNORING DX12 ENTIRELY, the 390 is still a better choice.
Also, you claim that by the time you'll use ASync Compute your GPU will be replaced. That may be true for you, but most people I know keep their GPUs anywhere from 3-6 years. Sure, I try to upgrade mine every 2-4, but if in 4 years my card is still going strong, why bother? Games using Async Compute are coming out this year, there will be more in the next 6 years. So for someone who can not wait for the next gen, it is certainly a consideration.
Also, really, buying anything but a 950 as a holdover for the new cards is really a bad idea right now IMO. You put all the other things the 970 has working against it and yeah, it boils down to horrible.