Good amount of biodiversity. Good amount of natural biological filtration. Long term health of fish is not of great concern. Occasional large fish losses are expected. No maintenance or technology required.
Energy requirement is huge, but provided by sun.
Everything is important to the health and survival of everything else. Old and weak become middlemen in the food chain. System does not attempt to maximize life span of individual animals. Some areas may have significantly higher stocking densities, but such areas are only possible because they are supported by the whole planet. To determine your stocking level, you need to take inventory of your pond.