It is a possibility for sure, but HD1 is still going after 8 years, and because AH doesn't host the gameplay servers, I reckon the server costs aren't excessive.
L4D2 is 15 years also, and still going, and still played. I hope and trust HD2 will be up long time.
It's Match Making isn't peer to peer, and requires a connection, though. I'd reckon the playerbase to wither quickly without Valve servers and/or Steam friend list, even though technically possible, and a 3rd party could devise a system of finding players.