Vacant, unmaintained lands tend to attract crime. I say this from first hand experience- I was a teenager once, and we went to vacant, unmaintained land to commit crimes. We were good kids, mostly, but we weren't the only ones there.
> by providing habitat for predators of urban wildlife, dense plant growth to improve air quality and soak up stormwater, etc.?
I'm all for having dense forests near high rises (more birds! more plants! yes!) but that land is expensive to buy, expensive to maintain, expensive in terms of lost tax revenue.
I simply argue that someone has to pay those costs, and getting mad at the city of being fiscally tight when politicians are elected promising exactly that is silly. You want to have a park/forest/whatever? Fine, but acknowledge that it's expensive and then pay your fair share of it.