Since windows 95', a sizable chunk of the entire history of modern games. The same way they learned how much money they made from having insanely crazy sales, they make money with different methods and ideologies. They're not making their own locked platform, they're building this on top of Linux and keeping steamOS…
While probably not completely open, I'm 95% certain this will happen and that Source 2 will be in line with the unity/udk content creation/indie development side. They have the distribution platform, the framework for content creation and consumption via user base...the logical step is having software to cover those…