Eh, I don't know. That would be an interesting introduction for people thinking about KF2 but it's already such a cheap game, especially on sale, that I'm not sure if it would make much of a different when it comes to population. Guess it's worth a shot?
I came here to post the exact same thing. It's sad to enter the lobby and find dozens of empty servers and no one to play with, at least in my ping range. I also think that making Killing Floor free-to-play is the thing that will revive it, and I think the moment to do this is now, since the game is dying, but not dead. Tripwire will be in people's minds and any game that come afterwards will have more visibility, you can profit with DLC's, weapons and skins to a large crowd on a otherwise dying game, and it will show a great deal of respect for your fans, who will have other people to play the game with, since KF is all about playing with other people.