Agree with Darlox.
Airline gets this revenue without incurring taxes on it. They nor any airline has any incentive (unless they want to reduce banked miles) of reducing income for change fees or bags fee in exchange for something that has no value if allowed to depreciate over xx months.
Would you prefer to be paid in CASH for something or given a dusty coupon which may not have any value to the owner due to its value and expiration date?