You are quite right, I wouldn't have thought it would fail. It failed in the ON position too which isn't what I would have expected although I guess I don't know why I would have favored OFF vs. ON. Maybe it arced and stuck closed?
The switch's MTBF was rated at 20,000 cycles I think from what I read. Maybe this poor amp was used more before I got it that I can imagine.
Also seems clear there was more than one vendors products used in that period.