It's absolutely fair to mock them for not releasing these games and keeping radio silence all these years. They managed to dethrone Duke Nukem Forever.
There were multiple times in which the internet was hyped for Episode 3 and where it would make sense to release even a basic game like they've did with Episodes 1&2 just to wrap things up. I'm sure plenty of people that make up various explanations to why that happened but the end result is that Valve has chosen to disappoint the fans who have been waiting for the conclusion to the story. It's not like doing that would prevent them from releasing an another new entry in the series that uses revolutionary new technology or whatever.