It would take many, many years for it to completely destroy the metal. While I would never do that to a good instrument, I would do it to crummy ones. Saluda ages their cymbals to order, and I am certain that they are not destroyed afterwards.
Well if there is one thing I am sure on its that there is a certain combination of chemicals that allows you to oxidize quickly and efficiently without destroying the cymbal.
So then one may say that cleaning a old cymbal can be a bad thing because the "patina" is whats left of the oxidation process and when you clean it you are therefore removing all that weight that was once there thus changing the sound maybe??