I don't get why steam doesn't implement GameSpy like features. That is, a common lobby that list (and allow you to join) public multiplayer sessions for all steam games that you own.
It would make it possible to revive quite a few of those "dead" niche multiplayer games. And greatly engage the steam users around the steam games.
Steam does have that - kind of. There's a "servers" window, which you can open from the View menu. It lists multiplayer servers for lots of different games - mostly Source engine games like CS:GO and Team Fortress 2, but also games with Steam matchmaking like ARMA2/3.
A lot of games are passionate creations and the creators would happily put a few hours in to report info to Steam servers if they thought it would double multiplayer use for another year or so at the end of a game's life.
It's the franchises which consider games dead after dropping the sequel, ie Call of Duty, FIFA etc. They want people to buy the next version every year regardless of whether the game is actually better enough to justify the cost.
It would make it possible to revive quite a few of those "dead" niche multiplayer games. And greatly engage the steam users around the steam games.