It was a gamble and a negative response from a lot of people was expected. Still, now that the idea is out there, chances are it will be tried again and may stick. The biggest problem with Valve's mod market idea was them introducing it with a game that already had a huge free mod market. Had they instead tried the model with a new game without existing mods, I believe the model may have stuck around (albeit with a big chance of it not being a huge success, seeing as most games don't have many mods even if they support mod making).