NYC will change and adapt. But from the perspective of someone living hear stuck in my apartment for the last few weeks, I wouldn't predict a mass exodus of older new yorkers. They are a pretty stubborn lot who live here for a reason. Maybe if we get recurring waves of large scale infections they would leave, but at that point, so would most people who could.
I would predict more wealthy families with school age children who have been considering the suburbs will likely leave, but a percentage of them leave every year.
I think one of the more likely and immediate impacts will be closings (as in going out of business) of many restaurants, delis, and bars and financial hardship for everyone who worked for them.
I got a request from a local restaurant I've been to exactly one time to donate to a GoFundMe to save the restaurant. I'm not sure what to say about that... Doesn't feel like a reasonable 'ask' to me. The place is good, but not sure my current priority for charitable donations should be a 30 seat restaurant with $35 entrees.