The problem isn't capacity. It's voltage. Old batteries are sometimes no longer able to provide adequate voltage at peak loads, leading to phones turning off when running on low battery (but before it's actually empty enough for it to turn off in normal circumstances). Until now I've assumed that this generally affects all li-ion batteries over time, but I'm honestly not sure why this issue hasn't cropped up in the Android camp in the same way. Maybe the only reason it became such a big deal was how Apple decided to fix the problem (silently throttling performance).