They are not losing any money, because Windows 10 is undoubtedly adware. It's "free" because you are the product, and they don't even try to hide this fact:

https://news.ycombinator.com/item?id=10053622 (a default install of Win10 communicates with third-party domains of well-known ad-serving/analytics/tracking companies.)

https://news.ycombinator.com/item?id=9976298

https://news.ycombinator.com/item?id=10053352

http://betanews.com/2015/10/15/microsoft-now-uses-windows-10...

http://www.extremetech.com/computing/204595-lock-screen-ads-...