This always happens with less well known online games. It really depends on how good the game is, TBH. Team Fortress 2, for example, (as in the 360 version) still has a load of people playing it, despite the fact that it's had none of the updates the PC version has had. Why? Because it's unique and extremely fun, and once you get past the "wtf y is ther no general machine gun class" (if you've been brought up on Halo and CoD, anyway) it's a fantastically unique and enjoyable experience. It's the same thing with Brutal Legend - despite the fact that the multiplayer was barely advertised and has a somewhat terrifying learning curve, once you get your head round it it's just so damn fun, and there really is nothing else like it.
So ask yourself this - is the multiplayer for Quantum of Solace a cheap, tacked-on extra (like Bioshock 2's multiplayer) or is it a hidden gem with a lot of potential for depth? That should answer how long it will live for - maybe not as a hugely popular thing, but if it's any good it will most likely retain a fanbase for a long time.