The Left is good at winning elections, what they suck at is having to rule effectively afterwards.
In a system where the Left can't subsequently take power, create a dictatorship or other means of ensuring they stay in power, they get a few years of rule, fuck everything up, and then get voted out. The harder they go Left, the quicker that happens because the bigger their fuck ups are.
I really hope Mamdani goes hard to the Left and does everything he campaigned he would do. NYC will get absolutely fucked for it. He'll get his term in office, maybe two, and then be forced out because the city can't take all the shit he's tossed on it. The alternative is it becomes another San Francisco and slowly dies.