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

[Sol]

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.

NYC doesn’t have extreme sorting. Maybe in Manhattan, but the broader city and metro votes similarly to what you would expect given the demographics.

If anything, many parts of the NYC metro are surprisingly red -- Staten Island is by far the densest county in the US to support Trump, and it wasn't even particularly close!

Yeah counties like Nassau and Bergen would be going 65-35 if they were suburbs of any other Northeastern city. They don’t stick out compared to the nation, but definitely in the Northeast.

Philly has a bit of a similar dynamic; a quirky characteristic of the non-New England northeast is the penchant of certain inner suburbs to vote Republican.