Valve said that there are no more updates this year for TF2.
Which says little, given that this year only lasts 6 more weeks.
On topic: While I haven'd played L4D yet I doubt it will be a "TF2-Killer".
Actually I don't think it works that way. You will always have games which will draw people away, but in the end it's always time that kills a game.
Back in 2000 it was Counterstrike which drew people away from TFC, but the scene remained strong, though smaller. It took another 2 or 3 years before TFC started to fade away with people abandoning it for other games or Real Life (tm).
However, what can happen is that another game will unhook you from a game like TF2 and after getting bored with that game you won't get hooked again.
This almost happened to me with TFC when I started playing BF1942 and, later, BF2.