If that's what you legitimately think of the developing world, then no wonder you believe in collapse so strongly (almost like the inverse of the "noble savage"; a fear of letting go of the "Western enlightenment" even if it comes from critics of modern Western regimes). Please, go outside and experience the world. Life has fundamentally changed for most of humanity, except for the occasional isolated remote community, over the last 50 years. I urge you to define what "peak prosperity" is and how developing nations lack it before we discuss what "collapse" is and the loss of "peak prosperity".
I have lots of family in developing nations and their lives have absolutely, fundamentally changed. It's borderline insulting to imply that developing nations still live like they did in the past.
Prosperity is always relative to the system you're immersed in and in a globalist world, your prosperity is defined by the prosperity of how people on the other side of the world live. Americans can believe they have the best living conditions in the world and same can do northern Europeans and maybe Japanese too. Let's throw in urbanized chinese middle class too. But in developing countries they don't have the privilege to believe they are in that competition. They are always a province of the empire and living in scarcity.
This says nothing about the enormous changes that went on pretty much everywhere in the rural parts of every country with very few, isolated exceptions.
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.
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.