New York City has been in a steep decline for a very long time. This will just serve to seal the coffin.
Texas and Florida along with so many other States are booming, and the surge will only grow.
New York City, LA, SanFran, Chicago, et al, are already places no one wants to be. It is difficult for companies in these places to get on-site employees. And nobody wants to actually move there. They have become like Detroit.
I do not think Mamdani will be much of a change except for pushing Islam in America as happened in Detroit. There simply isn't much room for these cities to get any worse than they already are now. When the Stock Exchange moves, which is very possible if not likely, that will be the death knell of NYC.