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I've bought an airline ticket (with SK) that I'm entitled to refund per their Conditions of Carriage because of a schedule change. They've agreed to refund me, but say that they will only refund me to the card I used to book the flight. This card has been cancelled, and I will not be able to get the money from my bank if it is "returned" to this card.

Are airlines entitled to only repay you to the credit card you booked with when this is not mentioned in their CoC? If not, who do you contact to enforce your agreement with them (short of small claims court)?

Call customer service and ask the issue to be escalated. I understand it is easiest and safest for them to pay pack on the original card but - particularly since there is no such restriction on the CoC - there is no reason to disallow another method of payment. Even if you did not cancel your card yourself, it is the same result if the card would be stolen or withdrawn from the market by the bank (last year on of my cards was cancelled because the bank stopped their relationship with the rewards program it was tied to, so it does happen). You can even ask for a check to be mailed.

Most airlines, and other companies, will only refund to the original card. This is done to avoid fraud, with people purchasing tickets with a stolen card, then requesting a refund to a card they control.

If the airline issues the refund to the original card then it will almost certainly succeed (despite the card being canceled). When this occurs, you will then be able to contact the bank that issued the card and request a refund. In some cases this refund will happen automatically, but it's still worth contacting them.

The other option is to have the airline issue the refund in the form of a voucher/credit that can be used for future travel. This would only be a viable option if you intended to fly with them again, and normally such a voucher would have an expiry date (often a year, either from the time it was issued, or from the time you purchased the ticket)

Your assumption of how the credit system works is a reasonable one. However it is also incorrect.

The system's refund procedure accounts for this situation.

If your card was reissued, the credit will appear on the reissued version of the card.

If you cancelled the card and closed and paid off the account, the refund will land as a credit balance on a closed card. It will sit there for awhile and then they will send you a paper check. If you want to hurry that process along, call them. If your address on file is no good and they can't reach you, it will eventually escheat to the state and you can get it from the state.

The refund can only reduce your balance, it does not substitute for a minimum monthly payment

If you owe a balance on that successor card and want the refund to go to a card with a zero balance so you can get one of those checks, that is one thing the system was designed to prevent. The merchant pays a fair service fee on your charge, and if he does a refund in the official way, he gets most of that back. If the merchant diverts your refund to another place, he loses that merchant fee.

I believe this may be to do with the rules governing the issue and use of credit cards (eg Visa or MasterCard Scheme Rules), and the T&C between the merchant (the airline) and their acquiring bank (the financial institution that processes credit card payments on behalf of the merchant). Taking an example from Barclaycard in the U.K., s3.13 of their Merchant Aquiring T&C states:
“Any refund must be made on the same card or account as was used for the original payment. You must not make a refund with cash if the original purchase was made using a card or account.” https://www.barclaycard.co.uk/content/dam/barclaycard/documents/business/help-and-support/merchant-terms-nov-17,0.pdf. Logically therefore, it follows that in using your card to purchase the ticket, you (and the airline) are bound by the credit card T&C rather than the airline’s COC.