Or, if the Github team, having first created a GUI for Mac, which they use themselves and can more easily test, were to then create a Windows version and charge

Or, if the Github team, having first created a GUI for Mac, which they use themselves and can more easily test, were to then create a Windows version and charge $0 for it, they could convince a lot of Windows Subversion users to switch to Git and Github, paying monthly for private repos. Which looks like where they're going with this.