The co-founder of Epic Games, Tim Sweeney, has spoken out in an interview and said that is worried about Steam on Microsoft Windows. Tim has made the prediction that Windows 10 is going to become restrictive and so limited so that it may end up making Steam turn from good to bad.
He said that there are two programming interfaces that developers have to follow. One of these is Win32 and this has been used by just about every game on the PC over the last few decades. Now though it looks as though Microsoft wants to convince people to turn to UWP and this could mean that Win32 is going to be phased out.
This may mean that software, apps and programs would have to be distributed via the Windows Store and it is going to turn the PC platform into a closed platform. This would break the library of Steam along with making it a great deal more tedious to play games.
If the prediction of Tim does prove to be true then Valve and Windows could part company. However, you should take this with a pinch of salt of course as nothing has been confirmed.