A few years ago, I went to Detroit to visit the ruins of Brush Park, as well as other historic buildings around the city. There was plenty of room for infill development. I'm certain that in several cases I saw entire blocks that were completely vacant.
I can't imagine that all that land has been purchased and developed. Even if it had been, the answer wouldn't be to develop parks.
Why not mow down all the homes that are abandoned, and offer those up for sale, rather than sell parks. I am sure there is more acreage available in blocks of unoccupied condemned houses, than there is available in parkland.