Besides nitrates, there are micronutrients in the water that get used up. I dunno, things like phosphate and other trace elements, that tank orgainisms need. Also, Other products of waste will accumulate, loosely described as dissolved organic compounds. Without water changes, buffers can get used up and cause a decline in pH. Our tanks are way more crowded than what the fish experience in nature, so that one could imagine how "stale" the water must seem to them.
The end result is that many have tried to make an aquarium a miniature self contained ecosystem with little or no maintenance and intervention, and have failed. Nutrients are always being imported into the tank, and not all nutrient decay will end up as nitrate. Just replacing evaporative loss is not a viable long term plan.
But, to say with certainty that X percent water change is inadequate is more a judgement than science, and will not be a correct statement in all situations. The combined and accumulated experience of AA users, acedotal as it is, strongly indicates that more water changes is always better, provided parameters do not fluctuate significantly because of them. Since Nitrate is a major waste product that accumulates relatively fast and is easily tested for, it is the primary way of assessing water change adequacy among aquarists.
