Obviously CGA is not intended to provide some kind of lifetime guarantee so the normal lifetime of the product is only relevant in terms of comparing warrantee terms between otherwise similar products.

I would say you would have to consider the price you paid and the way its intended to be used, because normal use of an interchangeable lense does involve some risks. Sigma could legitimately claim that they cannot be liable for customers treating their product a bit rough, so the CHA implied warrantee should only cover the time they expect it would take for a manufacturing defect or loose screw to become apparent. They might even agree that the diaphragm is a pretty robust part of the lense and just pay for it to be repaired or at least investigated.