Not in mobile. The largest increasing market for games is mobile right now. What's MS's marketshare in that? 2.4 %. Using Direct X today just doesn't make a lot of sense if you want to port to mobile devices.
If and whwn Linux gets high uality modern APIs and tools,maybe. it's currently way easier and less frustrating to get a modern renderer up snd running using Microsoft's APIs and tools. As a developer with a budget and timeline, that matters. The only thing Linux has which Windows is missing is Valgrind. Meanwhile, Linux is completely lacking in high quality GPU debugging tool, and OpenGL/OpenAL are simple awful APIs. OpenGL has an excuse in that it is ancient; OpenAL has none, it was just designed by morons who copied OpenGL. Which is why so few real games use OpenAl directly and pay for fmod or wwyse, and why game devs prefer D3D9/11 over GL. Better APIs save time and money, as do better tools.
With OpenGL, if your renderer doesn't work, good luck figuring out why. Way easier to get everything working on D3D/Windows and port after you know your game is corect and works.
Did you ever figure out why Valve was getting better frame rates with OGL?