In the wild there's hundreds of thousands of gallons of water per inch of fish. For now realise that this is a very closed system with very low water volume compared to fish. In addition, the waters our fish come from to have natural drainage and collection through streams, rivers, etc.This is going to sound stupid but Why did God make rain if it was not to change the water for the fish.
Assuming it keeps nitrates in check, it replaces trace elements, then so what if its not "new" water? After all, the air you and I breathe has been breathed a zillion times before, but its not somehow "used up" as a result.
Good point, and just as earth has trouble keeping up with depleting rainforests, in a heavily planted tank that naturally helps clean the water we have a variable that can help a lot.