What would you really add or change that would warrant a third game though? The second game can already be expanded into multiple full games thanks to the mod community.

The first game laid out the concept, the second game refined it and expanded on it. While technically nothing stopping then from making more of exactly the same game such would be disappointing against most people their expectation of improvement and would be easily criticized from all kinds of corners. Weather it is fair or makes any logical sense valve today is who they are part because of their imago, maintaining there respectable and high quality standard is economic survival.