Steam on Linux is not the only thing that will be a problem for Microsoft :
1) many people didn't upgrade to Windows 10... and it's not now that upgrade is not free anymore that they'll do it... Including people who did the upgrade then downgraded because of the issues with Windows 10. These people keeping old versions of windows will keep a good game performance and that can ba used against Microsoft to prove their malicious intent
2) Steam is also present on the MacOS/X platform... Although PC are the most present systems, there are many people under MacOS/X, thanks to (because of) the iPhone/iPad/iPod and iTunes. Microsoft is not the single player anymore OS-wise
3) WINE is getting better and better and you see more and more games/apps fully playable under it... including Steam and some Steam games.
4) Steam on Linux as a "device" (steamboxes). Their presence is small but they are one more alternative
5) of course, as you pointed out, Steam on Linux, which is getting more and more games...
Now, it's time that the game developpers start to embrace cross-platform technology like OpenGL/Vulkan, OpenAL, SDL,
Microsoft tried to stop WebGL because it was a subset of OpenGL and was relying on OpenGL, a technology they tried to put away... And OpenGL is also coming back as OpenGL ES for iPhone/Android... So having good OpenGL skills is clearly not stupid...