I really don't get the argument that people are trying to make here. New York is a blue state, sure, but it's substantially more Republican than Texas is Democrat, and Cuomo a.) has a 60 percent approval rating and b.) outperforms generic NYC Democrats among upstate voters. If you want to argue that Cuomo would outperform Hillary (or many other Dems) among upstate voters in 2020, these are the datapoints you would use (I would disagree with that argument, but still).
The last Siena poll I saw, showed New Yorkers want him to be reelected by a good margin, had Upstate wanting someone else by almost 20%.
I have no doubt that Cuomo would win New York and win it by close to 20% against Trump. However, I think Trump will do better upstate than he did in 2016 against Cuomo.
Nominating Cuomo, basically means that it is a Cities+suburbs+minorities strategy in 2020 and the hostile trends in the rust belt areas continues.