I don't know what levers he has to stop NYC from destroying themselves. If Mamdani manages to get the city council to defund the police for example, there really isn't anything Trump could do about that - at least until defunding the police results in chaos. If Mamdani gets his wish and create government run grocery stores there is nothing Trump could do about that either.
There is still a good chance Mamdani does not win, although Cuomo refusing to drop out could actually get Mamdani elected. Even so, there seems to be a coalition forming behind Mayor Adams who is running as an independent.
Republicans gave up a golden opportunity to take control of NYC when they nominated Curtis Sliwa. That's a joke.