Oh, economic rationality is a pretty weak constraint. Keeping your parks makes perfect economic sense---you'll just have to be aware that they are part of your consumption spending and account for them as such.
(And ultimately, all economic activity is to produce consumables one way or another.)
Moreover, even if some powerful lobby were able to show that it will derive modest benefits from bulldozing all of Seattle's parks and replacing them with temples of consumerism, it doesn't follow that doing so would make economic sense for the people of Seattle or the people of Washington in the main, once negative externalities are taken into account. It's not that you can't put a price on nature, it's more that humans don't do a good job of it when they even try, and law that takes this fact into account is good law.
(And ultimately, all economic activity is to produce consumables one way or another.)