Excellent article, and spot on that green spaces are still necessary in urban settings.
Having formerly worked with developers, I can tell you the only reason they're building on greenfields and asking for "more land" is so they can maximize returns. Brownfield sites usually cost more since they are closer to urban cores, have something on them that need to be demolished, and require additional remediation of the soil (burning it, in the case of former gas stations). Plus, single family homes are cheaper to build than anthing that requires fire stairs and a lift, so, again, a higher return.
The only way to conbat them is regulation, which is hard when the populace is demanding more housing and still idolizes "a plot of my own". And when cities forget they need to pay for the infrastructure to support them.