You are assuming that a product designer needs the product to last as long as possible given our current knowledge of physics, chemistry, engineering, and manuf

You are assuming that a product designer needs the product to last as long as possible given our current knowledge of physics, chemistry, engineering, and manufacturing at the moment. Most of the time, that's just not necessary. Things break, and if you can make some money off of them before they break then we can keep the cycle going. Customers would happily spend the same amount of money again after some time if they expect an improved product (for proof, see every subscription service).