If the PCs must be the beat cops, so to speak, in a bad neighborhood, well that opens up a lot of conceptual space, but at some point it is quite likely at least some of them will move on beyond the boundaries of the game concept and it will end. There's also likely to only be a certain amount of stuff you can do in a really narrow premise/milieu before people have started to run out of things to do. Its like cop shows, they start out with just 'cop stuff' but by the end of season 2 all the character's girlfriends, kids, the mafia, whatever is all wrapped up in it and it starts to break down, or at least you have to kind of reinvent the premise to keep it going.