batteries have a 100% failure rate aswell.

pretty much if it stores juice it cant do so with out some sort of internal wear unless completely discharged and even then its possible that it will leak or corrode.
just part of the process.
"super" caps have the same problem just a longer life span, basicly what capacitors would have been before if they weren't inherently manufactured in mass with a defect in the blueprint and/or chemical composition for so long