It's best for everyone except Microsoft if DirectX 12 goes away and developers avoid it whenever possible. Windows 10? Thanks, but no thanks.
Here's why...
We get it, MicroSoft bit you when you were a baby so you hate them with a passion and Windows 10 is the final sign of the end times, enough already. Vulkan is all well and good but it's not actually supported by the OS most people have on their computers. DirectX is the API used most commonly by game developers for a reason, regardless of whether you like it or not. 'Nix lost this battle over a decade ago due to not being willing to cater to the customer base, open source stuff is great, I wish it actually would take off and get MS out of the market, but until open source caters to the idiots who buy computers and phones and tablets, it ain't happening. Remember, the average 'Nix manual is a few thousand pages, while the average Windows manual is a few mouse clicks...people are on average rather stupid and lazy, computing that requires thought and ability doesn't sell. Bill Gates is a great man, but I for one kind of wish his parents had been pro-birth control, ya know?
Devs are already in the process of upgrading to DX12 or using it as their basis for new games, fact. It seriously increases performance without increasing overhead, fact. It took 2 devs 6 weeks to convert Age of Wushu to DX12, fact. It's only complicated and a pain for those devs who insist upon keeping DX9 support, if you aren't concerned with that(and no game released in the past 2 years should be!), it's not an issue and it's not all that difficult, see Age of Wushu conversion, but again, loss of DX9 support results in upgrading to support DX12, and again, nothing released within the last 2 years should have a problem with that loss.
Horizons for example, no DX9 support, you must have DX11, FD have said this, so upgrading to DX12 wouldn't be an issue, and the performance increases while dropping the overhead would benefit everyone, even those who are still only using DX11, due to the optimizations done when upgrading to 12(Chris Roberts talks about this in an interview recently, CIG is upgrading SC to DX12). This is also why Mac users won't get Horizons, the OS doesn't support DX11. Personally, I'd be surprised if FD isn't actually looking to go DX12 after the release of Horizons, they are already moving to a pure DX11 state, may as well go DX12 and get the performance boosts while cutting the overhead. And CIG is doing it for SC, that'll definitely put a bee in someone's bonnet at FD.