You can count on a single hand the number of office towers erected in the city in the last 20 years. Even if you believe Philadelphia is on the cusp of a meds-and-eds-fueled expansion, it's going to take a long time before Philadelphia attracts enough companies to fill the millions of square feet in the proposed innovation zones. Unless a Google, Apple, or other large, out-of-town tenant can be persuaded to open a Philadelphia office, there will be no iconic skyscraper, predicts Glenn Blumenfeld, a tenant broker at Tactix. It's just too expensive to build high-rises.