And anyway I think you'll find it just as 'impossible' to do any zoning change that reduce any asset on the mortgage holder's books: Anything that actually reduces land price is equally politically 'impossible'. One may as well try do something that is going to work broadly and fairly rather than a random patchwork of small superficial zoning fixes.

And incidentally it's probably easier to argue for fairness and fiscal responsibility than to argue for bulldozing open space to put in a high-rise apartment.