He is a licensed electrical engineer with years of experience and he is not wrong. There is no button to press, just a lever to move and he examined the insides and pronounced them in perfect order and they would stay that way. All he did was blow out a bit of dust in there. If this man says it, one can believe it.
Nevertheless, metal springs can and do break - especially very old metal springs.
also an ee. levers can break, toggle switches can break, inner springs can break, the connection/contact points behind your plastic turning dial could break. the probability they can break on an old unit, sure, are far less than a newer one that has a circuit board. Not impossible.