What if a farm is able to "produce perpetually", yet has negative effects on environmental quality elsewhere? Most people concerned with sustainability take a global view, so they try to avoid negative off-farm impacts. For example, over-application of synthetic fertilizer or animal manures can pollute nearby rivers and coastal waters.

On the other hand, if crop yields are too low, because of soil exhaustion of nutrients or reduced ability to retain water, farmers would need to access new lands for agriculture, leading to the decimation of the rainforest, draining wetlands, etc.

Cite: Wikipedia