I don’t have a natural pond it is the same as yours rubber liner, not all of it is bare bottom though just the deep portion. I strive to have the pond in balance with minimal interference from me. I have had my current pond for 10 years with zero fish deaths from disease and the pond I had at my previous home for close to 20 years Had pretty much the same result excluding the learning curve from the first few years. Did I lose fish yes but from predation. Don’t do water changes, don’t add koi clay, just weed plants, empty skimmer baskets, rinse filter mats in skimmer as necessary. Have koi, orfes, goldfish, and catfish in my pond along with turtles.
If a pond is under a 1000 gallons then I would agree it is more like an outdoor aquarium or if it is a DKP no matter the size then yes, water changes will be necessary to maintain water quality. However, large ponds that are set up to filter with plants (eco ponds) will not require water changes if done correctly. Why because the fish are getting nutrients/minerals from the food they naturally forage on in the pond, bugs, worms, plants and other aquatic life, as well as what we feed them. The plants are pulling nutrients from the water from waste created by the living animals in the pond, which along with the various bacteria in the pond, keeping the pond in balance. The whole point of the eco pond is to establish a food chain from phytoplankton all the way up to the top, our fish and keeping a balance.
I am not saying fish and plants don’t need minerals and nutrients to survive, just disagree on water changes being necessary to provide it and maintain water quality in all instances. Since most don’t test their source water how do they know their tap water is actually providing essential minerals or is reducing the nutrient level in their pond. Not saying your way is right or wrong, I always believe pond keepers/aquarists need to do what works for them, regardless of whether I do it or not. Nothing wrong with differing view points on how to do or not do things, there are many different approaches to pond keeping but individuals need to know and think for themselves so they can make informed decisions.