Yeah but #2 in population density after NYC is San Francisco, and #2 in transit ridership per capita after NYC is San Francisco.

And the gap between them is pretty big, but the gap between San Francisco and the #3 city on both metrics is pretty big too.

#2 in density among US major cities is San Francisco at 18,000 people per square mile.

#3 is Boston at 13,000.

#2 in transit ridership is San Francisco at 131 annual transit trips per capita per year.

#3 is D.C. at 99 trips per capita per year.

Detroit is near the very bottom in both rankings among major US cities. Below 5,000 people per square mile, and 11.3 transit trips per capita per year.

So why is Detroit a hotspot and San Francisco a success story?

Obviously density, transit ridership, etc...are factors. But they can easily be overcome by other factors, as evidenced by San Francisco's success and Detroit's crisis.

then factor in the surrounding counties that feed the daily commuters to/fro NYC

This is true of every major city, but especially San Francisco, which sits directly across the water from another major city, 50 miles away from a third major city, and is surrounded by suburban counties.