Jane Jacobs, in The Life and Death of Great American Cities (Random House, NY, 1961) writes:

• Age of buildings, in relation to usefulness or desirability, is an extremely relative thing. Nothing in a vital city district seems too old to be chosen for use by those who have choice -- or to have its' place taken, finally, by something new. And this usefulness of the old is not simply a matter of architectural distinction or charm . . . In successful districts, old buildings "filter up." . . . Some people, for instance, prefer more space for the money (or equal space for less money). P. 193

• The self-destruction of diversity can happen in groupings, in streets, or in whole districts. The last case is the most serious. Whichever form the self-destruction takes, this, in broad strokes, is what happens: A diversified mixture of uses at some places in the city becomes outstandingly popular and successful as a whole. Because of the location's success, which is invariably based on flourishing and magnetic diversity, ardent competition for space in this locality develops. It is taken up in what amounts to the economic equivalent of a fad. 
  
  The winners in the competition for space will represent only a narrow segment of the many uses that together created success. Whichever one or few uses have emerged as the most profitable in the locality will be repeated and repeated, crowding out and overwhelming less profitable forms of use. . .
  

Thus, from this process, one or few dominating uses finally emerge triumphant. But the triumph is hollow. The most intricate and successful economic mutual support and social mutual support has been destroyed in the process. Pp. 242-3