I think he doesn't expect the rich will leave because its NYC, business capital of the world. He's that arrogant to think the rich won't leave or build their businesses elsewhere like Ken Griffin is doing. Mamdani lost a $6B investment in the city and decided to expand in Miami because of his commercial.

His tax still has to be approved by the State Legislature. If its enacted, then comes the legal challenges from the rich. In the end, if they don't like it they'll leave. Really, the pied-a-terre tax is only the beginning of taxation. Of course the socialists like it. They think they're entitled to that money.