You said it yourself, free beats everything else. For the vast majority of people the OS is free, they either buy a Mac (OSX already installed) or a Windows PC

You said it yourself, free beats everything else. For the vast majority of people the OS is free, they either buy a Mac (OSX already installed) or a Windows PC with Windows already installed. So installing Linux is actually not free (it requires extra work). I would actually not be surprised if the market share of Linux exceeds the market share of Windows where Windows was installed by the user (let's exclude corporate environments because the discussion is much more complicated)