The problem exists with almost every Xbox Live game save Modern Warfare and Halo. 
At what point does a game just have so few players that playing is no longer an option? 
 
As of right now I haven't bought Monday Night Combat but am looking forward to the time when I can but I worry about the fervor dying out before I get my fill. 
Thoughts?  

People buy new games and stop playing old ones, unless a game gets a huge fan base, or a strong cult following, this is the way of the world. Developers can stem this by supporting their games with updates, DLC or putting it on sale to boost the player base. The PC has the modding community, which adds even more support, but consoles depend only on the developers to add more content.  
 
It always sucks when you can't find players, my advice would be to ask some friends to play. 

I kinda hope MNC stays, it's pretty decent (EpicSteve told me on XBL to get it over Scott Pilgrim, I'd rather play Scott Pilgrim on an iPhone, that'd be awesome), Battlefield 1943 actually has a fair amount of players still. 
 
It's just the way of the world, TF2 is only going to die once Valve brings TF3 (doubt it) or they go crazy and fuck everything up with the updates (doubt it again). Call of Duty games just get new ones annually, they'll go shit once Activision starts releasing them every 6 months. Halo looks to come to a halt but Reach will make it live a few more years then Bungie will make more. Counter Strike still has players, new ones too, Counter Strike 2 would be loverly TBH. 
 
PC gamers have the edge of Mods and Servers keeping things alive, but with Piracy still at large (Steam is helping far more than anyone would have thought.) PC games are slowing getting fucked. 
 
My two cents.

" People buy new games and stop playing old ones, unless a game gets a huge fan base, or a strong cult following, this is the way of the world. Developers can stem this by supporting their games with updates, DLC or putting it on sale to boost the player base. The PC has the modding community, which adds even more support, but consoles depend only on the developers to add more content.   It always sucks when you can't find players, my advice would be to ask some friends to play.  "

The population will inevitably decline a great deal, especially once the big name shooters like Reach and Black Ops launch later this year, but I don't think it will ever get to a point where you can't find a match because of too few players online. There are plenty of shooters out there that most people would consider to be "dead" (like Shadowrun or Team Fortress 2) that still have enough people playing to find a quick match at almost any time of day.