You're still missing the point of my question. I was concerned with the scientific reasons behind the instability of developers. By *any* measure, most are very unstable materials. I formulate highly oxidizable products that can sit in an oxygen environment for years sometimes without degradation. Because of my background I wanted to learn why a photographic developer cannot do the same. I learned that from this thread, regarding why only weak anti-oxidants can be used otherwise you get no development activity. The developers get diverted towards reacting with the anti-oxidants, not the silver halide.
Cost to the end user had nothing to do with my question. I was thinking and asking from the point of view of a formulation scientist. I make products for customers and they *demand* a long shelf life. Any product that I formulated that went bad in 6 months would be a complete failure. Replenishment is simply replacement of key additives. That is not stability, that is replacement. Would you consider your transmission fluid in your car highly stable if every 3 months you had to go to the shop, pull out 25% of the volume and "replenish" it? No, I don't think you would.
As I already said, I know all about methods to improve stability from a hobbyist point of view. That has nothing to do with my scientific query.