Okay, it's not a fully closed system. But where do new users come from? Without actually building much new residential space, or improving access from existing neighborhoods to new parks (via, for example, rapid and direct transit) the city's parks have a largely fixed number of potential recreational users. Are that many people going to make day trips into the city because of the existence of the Greenway as opposed to the existence of Boston's already established parks?
The dynamics of commercial real estate speculation are obviously complicated, but in theory, office towers are built to accommodate already percolating economic activity that will move into them. Developers don't expect the construction of new towers in and of themselves to generate that much new activity. And developers don't think of a city's office space as a total system; they don't care if an older building becomes abandoned and has to be creatively reused because their developments sucked the life out of it. Parks are a different story.