A poster on the SkyscraperPage forum recently presented the idea that as more people move to the downtown of major cities, these downtown areas actually become less lively than before. While this sounds completely illogical (you would assume that the more residents in an area, the more lively it would feel), he (and several other posters in the forum topic) do present some solid arguments. Perhaps the number one argument to this idea is that residents require a quieter space to live. While any major city downtown is not going to ever be the quietest place to live, residents of any area do demand (and to a degree, require) some sort of peace of quiet where they live. What this means is that while a downtown area will never be void of livelihood, it is very likely that the noisier, more active areas of a downtown will become very disjointed from the quieter, more residential areas of the downtown. And this, in the end, is actually a very rational argument.
You can see the full forum topic here
