"I think rusting of the tins is such a rarity (it may happen, but seldom, and not much, and even if it happens, it has not much significance"
I beg to disagree. Maybe it is a "rarity," I don't know; but I've encountered it with several tins I've acquired. And "it has not much significance"? You're kidding, right? A tiny dot of rust on a tin can compromise the seal, causing the tobacco to completely dry out. While it can be rehydrated, you'll lose forever any additional topping that might have been added to the blend. Rehydrated tobacco will never taste as good as it would if that seal hadn't been lost in the first place.
Bob