The misconception is that SK is scheduling flights which it "knows" that it will not operate. This is not likely the case. Rather, just like most carriers, it has a notional schedule which it would operate but for travel restrictions. As those restrictions become more clear for a time period, decisions are made.

There is no question but that if SK cancels a flight, one is entitled to a refund, thus if one accepts a voucher, that is up to the passenger. For me, accepting a voucher is always a poor idea unless there is a substantial premium or other benefit conferred. If the voucher is for the face value of the ticket and there is nothing else of value, cash will always be better.

For those who use credit cards issued in countries with absolute bankruptcy protection -- including the UK and US -- the risk of a voucher is even greater. If a carrier is unable to perform through insolvency, the funds are credited relatively immediately. A voucher, on the other hand, has no value unless some other carrier purchases the carcass and chooses to honor vouchers as a means of building its brand (unlikely in this environment).