You misread things, and badly. Mamdani's plans are only just now getting started. The flight is not going to happen overnight. It didn't happen that way elsewhere. But it did happen. It could take 10 years or more to really see the effect from this, and in part that'll depend on how long Mamdani is in office and who succeeds him.
The "Rust Belt," the fall of Detroit "Motor City," the shift of what was then the 'Silicon Valley' of the East Coast to California, and then a second shift of it to Arizona and Texas, didn't happen in a few months. It took years of bad government mismanagement and policy to drive industry and The Rich away.
In Newport Rhode Island, there's a stretch of road called "Millionaire's Row" from the time of the Robber Barons. There are huge mansions from that era scattered across the state. Today, they are more historical curiosities than occupied residences. The money went elsewhere.
Mamdani and Hochel are doing the same thing in NY. The Rich may not leave tomorrow, or next week, but they're already thinking about it and it's going to happen if the Left keeps up as they are doing now.