You might have heard of Microsoft's latest plans (source) to keep people on their own store, with a locked down Windows 10 S mode to be available on all versions of Windows. This is easily a first step towards Windows 10 S being the first version of Windows that users see.
Windows 10 S is essentially a version of Windows 10 that's locked into the Windows Store with Universal Windows Platform (UWP) apps, so you can't really run traditional applications like Steam and so on.
This goes directly back to how Gabe Newell of Valve and plenty of other developers felt about Windows 8. With Newell saying "I think Windows 8 is a catastrophe for everyone in the PC space.". There's also Croteam CTO Alen Ladavac who wasn't too pleased with it either, he's now tweeted about this latest issue from Microsoft to say " 'I told you so' doesn't quite cut it. :P". Ladavac also said in a reply "Think about it - if apps need to be adapted for UWP, it might be wiser to just adapt them for OSX/Linux instead.".
It makes sense too, if Microsoft is determined to make Windows more locked-down over time, that's not really good for anyone. Actually investing into Linux gaming, where you have far more control opens you up to many more opportunities.
Apparently, Windows 10 S can be upgraded to a "normal" version of Windows 10 Home for free, but the problem is that Microsoft has said around 60% don't even bother to do the upgrade keeping them locked into the Windows Store.
I hope Valve is keeping an eye on this, and it should certainly make Linux and SteamOS quite attractive again for them. There's good reasons why Valve has kept SteamOS around and plans like this from Microsoft (even if they fall through) will happen again and again. If Microsoft fail, they will wait a while and try it another way.
How long will it be until you have to pay to upgrade to Windows 10 Home, how long before the Home edition doesn't exist? Many questions—questions which should probably alarm people.
Thanks for the tip kellerkindt. Note: Article intro updated after publishing to better reflect my own point.