There's a huge difference between D/R+15 vs D/R+90

[Skill and Chance]

New York certainly would be more competitive if NYC was only D+15.

Exactly. NYC being the nation's biggest city isn't what inherently makes NY so blue; one could argue FL and TX are just as urban, but metros in those states are generally much more conservative.

There must be some self-selection factors involved. One can say the self-selection factors in New York's case are among the most heavily pro-Dem of any big metro in the US.

The thing with NYC is that it's so big and so diverse that it isn't really that selected.  It doesn't clearly  have a type, in the way that Boston or the Bay Area do.

[Skill and Chance]

New York certainly would be more competitive if NYC was only D+15.

Kinda.  If you shift the five boroughs to D +15, Biden still wins New York by ~9 points.  That's a lot closer, but it's not exactly "swing state" territory. 

I think the implication, though is that in a world where NYC is only D+15, Long Island would be voting like Montgomery County, TX, and Westchester like Forsyth County, GA.  That presumably would flip the state?