One of the major things about Windows 10 that has shocked the industry is that Microsoft has yet again given away a basically free operating system. Windows 8 and Windows 10 both were basically free (except for the $40 for the Windows 8 upgrade) and Microsoft is not feeling any pain. However, Windows 10 might bring in more revenue for Microsoft in ways that most users have overlooked.
Windows 10 apps such as solitaire now come with advertising (non-malicious or privacy violating) as opposed to being completely free of advertisements like in the free versions. Not to mention the renewed focus on the Windows App Store could potentially bring Microsoft more revenue from both app sales and advertising within the apps.
Since Windows 10 works across all devices it also means we can expect more users to be making use of the same apps and it is more appealing for developers to make 1 app that works across all platforms. The new focus on Windows apps should return any potentially lost revenue from operating system sales.
A common rumor at launch was that Microsoft would charge a subscription which was never true and was misinformed by other news outlets. However, with the new push for advertising and apps Microsoft does not have the need to push a subscription model for its operating system so long as it can gain developer interest and strong advertising partners.
Microsoft also needs to take greater steps to keep their app store clean of malicious apps and other issues that might deter users. But if Microsoft can keep all of these in check, then they will be generating more passive income in a new revenue stream that was almost entirely foreign to them 5-6 years ago.