Developers admitted taking a “leap of faith” when they pitched a $2 billion transformation of five blocks of parking lots and industrial buildings in Astoria, Queens. The most publicized aspect of their risk paid off: The City Council approved the rezoning they needed to build more than 3,000 homes. But the future of the project is uncertain given the expiration of 421a, which freezes developments’ property taxes at pre-construction levels for 25 years and discounts