Drains and Maintenance
A drain is a feature that takes a resource out of the game for good. Decay is the usual drain in construction and management simulations: Buildings or other entities wear out and have to be replaced, which costs money. In SimCity, for instance, the roads wear out and have to be repaved. If the player doesn't repave them, the sims start to emigrate because they can't get to work.
The player has to manage the repaving in SimCity personally, but in many games, these maintenance tasks are automated so the player only has to pay for them without actually performing them. Maintenance annoys some players, who would rather buy something once and never have to worry about it again. However, maintenance is an important game balancing tool; it drains resources and prevents the player from building profits endlessly. If you characterize maintenance as an ongoing cost rather than a purchase of assets, it makes more sense. Paying employees is a maintenance cost. You can't own employees, but you have to pay their wages on a continuing basis; if you stop paying them, they stop working.
You may want to give the player the power to turn off or adjust the level of automatically managed maintenance (and suffer the attendant consequences) so that he can make use of the money for something else that he needs in a hurry. Stronghold 2, a game about managing a medieval castle and its inhabitants, allows the player to set his peasants' food rations to one of five levels: none, half, normal, extra, and double. With these settings, he can manage the peasants' food consumption, which is one of the drains in the game.
Decay is a continuous drain that the player may or may not have to act upon, depending on whether you allow automatic maintenance. For a more dramatic effect that forces the player to act, you can include disasters. SimCity puts more pressure on the player by having fires, tornadoes, and monster invasions crop up periodically, doing considerable damage. If the player does not take action to repair the damage (which costs money), the city dies, not just through the destruction of buildings, but also through the loss of needed infrastructure such as roads and electrical lines. Disasters need not be natural ones, of course. In a good many games, the disaster is invasion by hostile armies.