Many things kill batteries. Grid corrosion, water loss, (dryed out electrolyte) as well as plate deformation and sulphation and the fact that the active plate material gets used up, voids and shrinkage of gel, Oddball chemical reactions from impurities, (they are black/grey/white dust, and obviously buggered...
The idea that you can fix a battery by getting rid of sulphation alone (which doesent work anyway) is pretty much a non starter. They even age and die just based on time alone. If you ever cut an old one up you would see the horrid corrosion, disclouration, distortion, etc and they just look knackered! The idea of rescuing them with ac or pulses of current is to be kind a bit "hopeful"!