Philadelphia - historic as it may be - has always functioned as a working city and as a result, has no inherently true "historic districts". Center City's one seemingly historic district is the result of a mid-century attempt to reconstruct a Colonial past, one which is only as important as a number of other movements responsible for the PSFS Building, the Divine Lorraine, even the Cira Centre. The result, Society Hill's "historic district", is a collage of questionable reconstructions which sacrificed dozens of 18th and 19th century buildings, some by Willis Hale and Frank Furness. This attempt at architectural cohesiveness created a very peaceful, historic illusion, but compared to the rest of Center City is one of the less interesting neighborhoods to look at.
Historic districts are important but it is just as important to respect the existing history of a neighborhood which has naturally evolved. The Keystone National Bank Building is a prime example. Is it more historically respectful to replicate the original facade which was replaced less than ten years after it was constructed, or do you pay homage to the five successive facades implemented over the following 100 years by designing something truly modern that represents the needs of the existing urban fabric of a culturally, historically, and architecturally diverse neighborhood? Unfortunately we usually fall somewhere in the middle, attempting to appease the devout advocates as well as the needs of the client, and we end up with bland, historic interpretations. Instead we should be replacing the avante garde masterpieces we've lost over the decades with exciting new architecture.
An empty construction site or blank facade has the potential to be architecturally significant someday. In a city as aesthetically diverse as Philadelphia, architects should be creating tomorrow's history and not wasting their time recreating yesterday's.