Yeah, I don't think it's about my opinion of people, it's basic math.
The Amish farm supports 100 people nice and happy, with a surplus. After the power goes out, people hang out in Philly for a week or so, waiting on rescue or conditions to return to normal. After that, you've about eaten everything you've got laying around, and grabbed what was nearby. Now what?
one day, a hundred people wander onto the farm. The Amish are nice and friendly, they suck it up and try to help. Next day, 500 more people show up. Now things are REALLY tight, and no one has enough. Guess what, 1000 people show up the next day, and they're even hungrier than the first 600 people. Well, Philly has 1.5 million living in the city, and that isn't counting much outside city limits. How long before EVERYTHING is gone from that farm? Every last bit of food, everything? At some point, you're just overwhelmed, and it starts to turn violent when things run out. People are also going to start getting the idea that leaving is a good idea, and the tools are gonna start vanishing (the animals have long since been eaten). Farm is eaten clean, that many sick and starving people have pretty much fouled the land, and there's just nothing left. Now what? And bear in mind that's just the first couple weeks after the blackout, maybe a month if there were a lot of supplies nearby to eat through. This is when the REAL carnage starts. Depending on time of year and physical location, winter will probably take care of most of this, and population will literally be decimated, if not more, by spring. It's not a low opinion of people, it's just basic math. The areas where most of the population are CANNOT support that population density, not anywhere near it. Your food comes from areas a thousand miles from where you live, and the high yield they enjoy is mostly because of heavy machinery. Take both of those away, and people are going to die fast, and in huge numbers. Smaller farming communities in the midwest are the best off, as they'll have food already out in the fields, planted pre-blackout, and no where to ship it to. They'll have the best shot at seeing the spring, if they can get the hands to help bringing in the harvest without losing most of it. And IMO they're just hardier people, and more used to living off the land and having useful crafts.
Katrina's not a bad example. Imagine the same example, but all the cell phones, flashlights, and generators are also dead. Government isn't showing up to help, no boats, no helicopters, no buses, no FEMA supplies. Now imagine that going on a month, instead of a week. How long before it burns itself out completely? And if it's happening EVERYWHERE, and there's nowhere to escape to?
I don't picture people living in poverty in the current setup ANYTHING like what we're talking about. When EVERYONE is in this situation, and there's no help coming and no hope of it improving/going away, whole new ballgame. Population density is the other part of the issue. They don't live in MASSIVE numbers in those conditions. And those areas that have larger populations STILL get things trucked in to feed/water them, even if it isn't enough. Suddenly apply the conditions in Somalia to NYC and you think it'll be ok?
Turn the question around: how can you honestly think it WOULDN'T be like we're describing? People are gonna shrug it off and sing songs? Just quickly adapt? How many people live in Atlanta, vs how many do you think the immediate area outside of it could support? And that's assuming they all immediately chip in and help, all have skill and luck with the farming, and you can plant stuff in that paved-over clay without a water source, hoses, etc. And everyone's going to share, no hoarding, no violence, etc. Think my situation is unfortunately more likely.