Uninhabited Brussels

When we talk about little or uninhabited neighborhoods in Brussels, we think first of all of industrial zones, large parks, or even green neighborhoods of the 2nd ring; in short, parts of the city that are little or not built. We think less often of densely built neighborhoods, in the center or in the 1st ring: yet, some of these neighborhoods are little, or even very little inhabited. The European quarter, the Royal quarter, and the Mont des Arts are among these. How did these neighborhoods "empty" themselves of their inhabitants? What are the problems posed by the concentration of a single type of activity (offices, administrative functions, or even "museum zoning")? What are the plans and projects aiming to reintroduce housing and mixity there or, on the contrary, to reinforce monofunctionality there?