> ...thus my observation that cities are not the center of the universe - they depend on the whole ecosystem just like everybody else.
It goes both ways: cities depend on farms and farms depend on cities. Neither can sustain its way of life without the other.
>And, here's a secret, every farmer has land and can have a garden. They will eat no matter what.
Here's a secret: not every farmer owns land. There are about 6 million farm workers in the US and about 3 million owners of farmland.
About half of farms are owner-operated and about half have a landlord renting out the land to a farmer or farmers.
The landlords actually have fairly concentrated holdings, with roughly 8% of the landlords holding half of the rented out land, or about 25% of the total amount of farmland.
In a situation in which farmers refuse to grow food, these landlords would lose their rents and have a very strong incentive to A) kick off the farmers who refuse to grow and B) get someone to grow food since the prices of foods would have gone through the roof.
These farmers will starve.
> Sure they need that oil and gps, to produce enough food for 10,000 people. But they just need their hands to feed themselves. So no, they aren't going to starve, or even suffer very much.
They need oil and gas for things beyond food for themselves. Heating and cooking, for instance, require a good bit of firewood. In colder climates it can take many acres to keep warm. Even in the Southern United States it can get cold enough to kill during the winter.
Many wells are deep enough that they require pumps to get water out. Guess what powers them? Electricity. Since lots of farmers own land in places that couldn't be grown on without irrigation, such as the desert, things are going to start sucking really quick.
Electricity also powers refrigerators and freezers, which enable food to keep for much longer periods of time and reduce the risk of foodborne illness. Without trade, are these farmers going to have access to salt to cure meats? Are they going to know how to properly store food without any refrigeration? They certainly won't have the ice cutting industry of the 1800s.
Gas also provides the transportation to get medicine, clothes, blankets, seeds, tools, pesticides, and fertilizer. Without cities, farmers would also not have accurate weather forecasting, something that is quite important for them.
TL;DR I think you vastly underestimate the difficulties of subsistence farming without modern technology or the benefits of trade between cities and farms
Source for statistics: https://www.census.gov/prod/1/statbrief/sb93_10.pdf