I'm sure that DF will correct me if I'm wrong here but TF2 was only meant for 24 people as well and he found a way around that. I came from CSS where I played on 60 man servers so that was a big cut. I'm sure that DF will find a way around the 8 man servers.
If you think about it L4D only have 8 people for MP makes sense. The whole point of zombies against humans in any game, movie or story is that there's always a mass of them and a few of us.
I personally have been a major fan of zombie flicks and games since I was little. The concept of an evil threat that's already dead with thousands to blow up over and over again just appeals to me.