Charitably but precisely, the moral is 'depending on trade exposes you to certain risks if the market changes.'

Yes a similar thought occurred to me today. I am a specialized professional worker in a big city. If the demand for my services went away, then I would have to either change jobs; depend on charity; or starve to death.

There is a subculture of people who worry about this kind of risk (among other things); they are known as "survivalists" or "preppers."