> cheap and mostly occupied by young people who can't afford anything else, because they're incredibly undesirable ways to live, limiting their market rental price

Yup, do this. "Young people who can't afford anything else" in New York are a population comparable with that of Atlanta [1].

> no way that you can seriously argue that doing this for newly-constructed skyscrapers would result in units that can be sold at the price that the building owners are demanding

The buildings' owners are screwed. Their leverage constrains them. The question is whether the structures have positive value. Retrofitting to current code leaves most with negative value, i.e. it's cheaper to abandon. The question I was answering is if it's possible to economically convert these to housing for somebody. Under current code, the answer is no.

[1] https://www.statista.com/statistics/911456/new-york-populati... 20 to 24

[a] https://en.wikipedia.org/wiki/List_of_United_States_cities_b...