The real issue here isn't even ideology, and given Cuomo's record it certainly isn't perceived competence or ethics.
It is that Mamdani isn't one of the lads and Cuomo is. Mamdani becoming mayor won't lead to full communism or even a tax rate bump. It might lead to a small reordering of the power structure, with some people seeing their influence wane and their friends and family career plans being slightly curtailed. The seating chart will be altered.
It is mostly about that.