I used to live in a new mixed-use development. The builders built as much of the residential parts as the county would allow, and then basically quit. It's been the better part of a decade now and the place still doesn't include a single office or retail unit. A huge chunk of land just sits empty. I'm certain that if the county stepped out and allowed the developers do what they wanted, that land would instantly be filled with even more residential units and that would be it. (And given the demand for housing, that may not be a bad thing, but the result isn't mixed-use.)