Hasn't valve ran about 30 TF2 servers forever? and a bunch of L4D ones too? I can't really see that being the justification.
The difference being, those 30 servers were never enough to handle the entire userbase of TF2, but there were also dedicated servers running from day one due to operators actually knowing the game would be coming out and probably getting access to the dedicated server software ahead of time.
Not sure how
Left 4 Dead works. I assume lobby players always play on Valve servers, which there are way more of than for any of their other games, and which might be virtual servers that start up and shut down based on estimated load.
But
Left 4 Dead wasn't a free-to-play game. Those servers are being supported by the money people have spent buying the game. If
Team Fortress 2 still cost money, I bet Mann vs. Machine would be paid DLC, and justifiably so. I mean, I'd probably fork over five to ten bucks for it, at this point.