Permaculture encourages community, not hyper-individualism. It's about working with nature rather than against it, and directing the benefits of it to everyone.
But life will be hard, once the fuel and electricity run out, even the ones who are totally off grid with solar and storage batteries. Just basic stuff like screws, pumps, pipes, tools, not to mention medical care, pharmaceuticals etc.
The groups of people coming from a city to pillage a farm clearly have no idea what they're doing long term and in all likelihood wouldn't survive much longer. Killing and processing am animal isn't a skill most people have. They could easily ruin the meat and would almost certainly waste most of it. And what comes next, unless they know how to hunt as well?
All that said, I hope we never see anything close to this ever play out. No matter what the specific scenario that breaks down society for more than a few days, a lot of people would die along the way and who knows what would come next.
The whole scenario does seem unlikely though.
Collapse is a process, not an event. Collapse is happening all around us, yet we're not all turning into gangs of marauders. When catastrophes happen in poor countries, they don't turn into roving gangs of marauders either. People tend to work together, actually.
>likely only highly organized and well armed groups will survive
[extremely USAmerican voice]
Of course it depends on the size of your backyard, and how long, and critically on trade networks/community.