A decade is roughly the upper limit of specified battery shelf life for cheap batteries. Making the battery bigger doesn't really help much at that point where

A decade is roughly the upper limit of specified battery shelf life for cheap batteries. Making the battery bigger doesn't really help much at that point where it's limited by shelf life, not capacity. There are longer life batteries, but they're _expensive_, e.g. a Tadiran TL-4934, 1Ah, with a shelf life of 40 years will cost ~5$ in bulk.