[verall]

Either there's not alot of current going across that switch, or it's just a little lucky - just because that one is going strong doesn't mean over half of the same switch hasn't failed in the same application.

Or, if it's really 40 years old, "cheap" could be "really fat and expensive" today.

If you pick up an old "broken" power (audio) amp, the most likely reason for it to be broken is pots - switch - caps - FETs in that order.