In simple terms -

If a passenger buys a non refundable ticket then he/she is stuck with it even if the passenger’s circumstances change - can’t’ change time/cabin or get a refund

on the other hand, if a passenger buys a non refundable ticket and the airline decides it can’t/won’t fly that cabin/flight/time … well tough, the passenger can have a refund or accept the change

IMO, once an airline sells a ticket they need to be held to the contract as firmly as a customer is.