Microsoft makes little to no money from selling OS's to consumers (Us). They do however make millions if not billions from something like Office365, which is subscription based which is where I think they are going with Windows 10.
They want to push their app store, so what better way than getting millions of people to use Windows 10 and getting app developers to push stuff through the Windows app store, whether it be once of purchases, or small monthly subs they'll make a hell of a lot more money like that than they would ever do by selling Windows 10.
Let's not forget about them selling all the user info gathered over each copy of windows 10.
Microsoft are being incredibly smart.
