The vast majority of games out there are created on NVIDIA hardware. The fact AMD manages to get a victory at all in price/performance is significant.
The majority of top PC games are console ports, and the console sales still makes up much of the sales for many of these developers, which is why there are more AMD partner games than there are Nvidia partner games in this segment.
Worst case scenario for D3D12 is that it uses D3D11 calls so even though it's technically D3D12, it performs like D3D11. A lot early D3D12 games did this--especially games that released as D3D11 and were updated to support D3D12 (The Division comes to mind). Software that's developed for D3D12 from the ground up will see a significant improvement in performance and reduced CPU load across GCN-based cards and Pascal-based (because of async compute) cards. Maxwell and Kepler cards see a minor improvement (mostly because of reduced CPU load).
At this point games should be developed for Direct3D 12 or Vulkan exclusively, there is no point in supporting pre-Fermi an pre-GCN for new top titles, and pre-GCN cards dropped driver support a while ago anyway. Windows 7/8 support is probably the only reason to have legacy support, but if by doing so you have to design a bad engine, then you should only support the old API.
It's pointless to simulate the old API through an abstraction layer. Sometimes these abstractions can perform worse than the old API, because the new APIs are designed around a different approach. The point of the new APIs were to leverage lower level control over the hardware, and re-adding wrappers to abstract that away defeats the whole purpose. Utilizing these APIs properly requires entire new engines built from the ground up to leverage this new level of control, and since this is painstakingly hard, we probably wouldn't see any widespread proper adaption of these APIs anytime soon. Most developers will continue to use abstractions to deal with the new APIs, and might even continue to do so. Games are unfortunately mass-produced trash these days, not only in terms of recycling the same concepts, but also in terms of code. They are usually stitched together before the shipping date, and then they move on to the next title.