It was always a bad idea to have competing APIs on one platform. Look at the disaster we had before with Glide and OpenGL and DirectX. It was terrible. In some games you had multiple options and there was always a problem with them because the developer felt they had to support everything. When they dropped that and just used DirectX it was a ton better for compatibility and getting games to work IMO.
I'm not talking about locking vulkan to steamOS, but Valve locking you into their SteamOS when you move away from DX because lets face it on Windows DX12 will get the most support. You think it's telling that Intel and Nvidia are working with Khronos on the spec? Hardly...they sell hardware and want to be sure their hardware can work with it so they can *gasp* sell hardware to people who might use it. Tough concept eh? Nvidia wants people to be using their GPUs and Intel their CPUs. Plus it might even benefit Nvidia's Shield platform since it doesn't run on windows. You know, vulkan is not really solely about PC gaming and never was. Android and other mobile platforms will probably make use of it. The thing that I was talking about is Valve trying to break away from Microsoft because valve wants to remain in control of digital distribution for PC games. In 2011 Forbes said that Steam was responsible for 50-70% of the $4billion in downloadable PC games. They don't want to lose that and funnily enough, Microsoft isn't even doing anything to block them. MS is only offering an app store where developers can make games available to Windows PC, Windows Phone, and Xbox platforms. Universal apps can allow a developer to sell their title on multiple platforms with a buy once play anywhere approach. They aren't blocking Steam on Windows. Valve is acting like that's what is happening though. People say MS is a walled garden for PC gaming while praising Valve. Valve is building more of a wall with SteamOS for gaming on PC than MS ever has.
Think about the whole situation being discussed before you call someone a shill.
For many people who have been playing games on PC a long time, you couldn't load up a copy of Deus Ex or System Shock 2 for nostalgia on SteamOS. That would be a problem for some users. I think a major thing that DX has going for it is the vast library of games that could be played from years ago.
Your issues with Valve and their issues with Microsoft are an entirely separate point.
Nothing is wrong with their being both Vulkan and DX12 on the market. For Windows-only developers, they'll simply choose the one that performs the best while also looking great. That's why OpenGL failed to have a significant developer mindshare for Windows-platform titles. OpenGL was always behind Direct3D and had fewer eye-candy features until they caught up. The only developers who chose OpenGL were the ones who wanted to make cross-platform games.
Valve choosing Vulkan is not an issue at all. Why? Because those Vulkan titles will still be available on PC, because even with SteamOS, Gabe knows a significant portion of revenue will remain PC centric.
And he knows some gamers wouldn't mind the one-trick pony approach that consoles offer, while maintaining the hardware strengths of a full-spec PC. That's why SteamOS will have some mild success, because if the gamer only needs a basic laptop with Windows for productivity but wants a dedicated gaming rig, and is okay with a small back catalog and some limits to new game availability, they may be satisfied with one of those SteamOS boxes.
Vulkan has a strong chance of having more success on Windows than OpenGL ever did, and that should not bother you in the slightest. It is quite likely any Vulkan titles will be Vulkan-only, so there won't be option juggling. And guess what? AMD and Nvidia will have strong Vulkan support, I can almost guarantee it, and at the minimum, AMD will for sure (as they already have experience with the Mantle core that evolved into Vulkan).
Many developers may very well appreciate the idea of a multi-platform API that performs and looks just as great as D3D. If Direct3D 12 performs better than Vulkan or has more eye-candy features, then the outlook might be different. But if I were a developer, and Vulkan met my needs and allowed it to compete with the best performing and best looking titles, I'd go that route, because it makes porting to Mac and Linux that much easier. That also allows them to still catch the potential sales of those who do invest in SteamOS.
This won't hurt the PC at all, I don't get why you are vehemently opposed to Vulkan.
You mentioned Glide, but that's not even in the same group. That was locked down to one manufacturer. That would be like what Mantle was. OpenGL was basically always available to users of assorted video cards. Perhaps some early cards did not support it, or the drivers never did, but today OpenGL is universally compatible on Windows, it just isn't that great compared to D3D11. Some will argue about that, but I haven't enjoyed the views offered by any OpenGL-only title. I haven't seen what Source engine looks like under OpenGL, however, Source isn't top of the line in visuals anymore and the features that were exclusive to D3D10 and 11 when Source was still a producer of premium graphics, are now available under OpenGL. It is an old engine these days and OpenGL can handle what Valve had in it just fine.
And from what I can gather, Vulkan will be able to match D3D12. Perhaps it drops behind and more developers choose to eventually focus on D3D12 and either port later or not port at all. That is a bridge that may or may not be crossed in the future. For now, everything is pre-release so we'll just have to see how things eventually play out as titles become focused on these new APIs.