It would seem to me that many of the older buildings, 50+ years, would probably need significant upgrading to meet any of the current needs for office space. Buildings are not made to last forever, just a projected period of time. Certainly reuse is an attractive option but when the dirt cost escalates to a number that is unsustainable for many retail operations, reuse goes the way of dial phones, telegraphs, etc. Large retailers have models they use for opening new stores. Unique and boutique type spaced usually do not fit their models.
If'n you owned a 6 story 50 year old building and the city was telling you that you had to provide X amount of parking spaces, and need to upgrade your elevators / sprinklers / electrical etc on the one hand and a developer telling you that he will take the responsibility for the building off your hands at a tidy profit, what would you do?