If the same happened to New York, with the subway abandoned, whole neighbourhoods overgrown by poison ivy and the skyscrapers collapsing from lack of maintenance, "post-apocalyptic" would probably be a fitting word.
I am not sure how you can call such a massive loss of complexity and population something else than a collapse.
It’s the timescale that makes it. I’m thinking Methyr Tydfel in the UK as a point of reference for similar fractional change in the first roughly century of decline, but peaking in the Industrial Age.
If this happened in NYC, I’d expect (say) Queens to turn to rubble while (say) Manhattan is getting freshly built skyscrapers. Plucky adventurers with assault rifles and bows looting tinned food from a suddenly abandoned Whole Foods is more of what I expect during hurricanes than as a standard result everywhere all at one.
I expect that, much like every other community, the loudest “collapsers” are merely an embarrassing minority, and certainly I hear that the people who stockpile food and maintain a strong relationship with their neighbours roll their eyes at those who stockpile ammunition and do nothing else.