Why is New York real estate valuable in the first place? Because a lot of people want to live there.
Why do they want to live there? Because there's a lot of other people there, and those people can afford them various opportunities in their chosen field.
If New York becomes a wasteland of empty apartments held by overseas billionaires, none of the people that make New York a desirable place to live (and thus, a valuable apartment to own) are going to be there making it desirable (and thus, making the apartments valuable).
Everyone I know who's living in New York can't wait to leave. It's a miserable place to be unless you're a billionaire. These people are destroying the value of their own investments just by making the investment in the first place.