That doesn't seem convincing, for the expiry. The issuer needs to keep track of cancelled cards anyway, and they should be cancelling expired cards anyway - it seems like a huge security hole to keep expired cards in an active state and rely on an expiry date check to reject transactions.

I assumed that it's a leftover from early days, before online card checks, so the merchant can check that the card is still valid (besides checking signature and ID or whatever). When introducing online checks of the magnetic swipe, expiry date was part of the data transmitted, and early phone/Internet payment systems relied on reconstructing the data present on the magnetic strip, to "fake" a swipe. Then it just stuck from there.