I think you're misinterpreting. It's not Windows itself that's sinking, it's the customizability of Windows. As people who care about user control switch to other OS's, the demand for custom Windows drops. Even gamers may simply dual-boot or own two setups if they want customization or privacy — you don't need much in terms of an OS to simply start a full-screen video game.

By contrast, XP was very customizable. I used to like BBLean.