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

[Vosem]

Also, just because one area votes solidly for one party doesn't mean it's maxed out for that party, which I think is another misconception I also see "well WI rurals are already red so they can't get any redder/NOVA is already safe blue so it can't get bluer". Nowhere is truly maxed out until you start getting to +90 levels in this era of partisanship

Even then there might be more gains left to squeeze out through relative turnout levels; in 2022 the GOP did very poorly in swing areas but still kept margins not-awful through most of the Midwest (and outright good in much of the South) just through totally ridiculous rural turnout levels. It remains insane that IL-Gov swung right between 2018 and 2022 even as every population center of any significance at all swung left in a pretty urban state; but rural turnout was through the roof.

[Vosem]

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!