In 1950 detroit was the richest city in america., they were a shithole in maybe 20-25 yrs, decline can happen fast.
There's a tipping point where all the people who dont like what's happening - whether its demographics, crime, services, jobs, all of a sudden bail. Once that happens a city will crash quick. dont know where that is for a giant city like NY - but it will happen.
The city will live on, but the replacement 's will be of a lower caliber, they wont want to work, wont care about their houses/property, once the "good" people leave, it's over.