Once you make the mental connection of Thieves' Guild = Organized Crime, it becomes obvious that the the Thieves Guild would control most fantasy cities. In the real world, there are many examples of places where the line between government and organized crime is purely academic. Also, remember that the Thieves' Guild is a Guild; it doesn't just commit crimes, it also regulates crime. The Guild would probably only allow so many crimes to be committed in a given month, for instance, in order to not deplete the city's resources. Excess crime would not be tolerated. Also, being professionals, the Guild would avoid killing (because you can't keep robbing dead people). The Thieves' Guild would in fact be the de facto law enforcement agency of a fantasy city. (In the same way, the Assassin's Guild would investigate and punish unsanctioned murders, keeping down the overall crime rate in the city). In such a situation, there would be a great deal of collusion between the legal authorities and the Guild. This situation also creates many adventure opportunities, such as the PCs being hired to crack down on some outlaw gang that's robbing people without Guild permission.