"Every ounce of material that is taken off of that land came from somewhere"
And every ounce of material on a farm came from somewhere. So, it isn't a mistake to sell nutrients (in the form of food), it's a mistake to fail to replace them. Many of them will replace themselves with little effort on your part-- plants will fix carbon and nitrogen from the atmosphere, animals will synthesize vitamin D from sunlight, weather will bring water-- but others take more effort.
Many permaculturalists don't like to import materials, and there are many reasons for this-- disease, cost, labor. But I like to import material. So many nutrients are sent to landfills, where they can be of no use. So many nutrients are washed into the ocean, where they create dead zones miles wide. So many nutrients are burned into the atmosphere, where they contribute to breathing problems and climate change. So I like to capture some of this whenever possible.