I'm definitely not sure that all of Cuomo's voters automatically go to Mamdani, I'm sure he'll get some but not all. But the math for Cuomo to run as an independent, especially with Adams and Sliwa in the race, is also very very difficult.
Brass tacks, Mamdani would likely get a majority (if not a significant majority) of Democratic voters in an overwhelming Democratic city, with multiple other candidates fighting over the rest. The math isn't mathing.