Someone suggested that in the giant forum thread.
The thing is, the people who still play CoH are SUPER dedicated. I know people who spend hours a day doing world-related stuff, who run radio stations themed around and listened to in-game, people who RP... If they open-sourced it, the current player base would probably only need, say, three servers - two American, one European - and a few people dedicated to making sure that if something bugged, it got fixed.
I don't know anything about video game management or development though, so that's just my assumption. It could be that it's an unmanageable task by anyone but industry professionals.
But IMHO, it's been running this long (albeit, I hear, with duct tape and bubblegum) - it couldn't take too much effort to keep the servers lit up.