Taxes?
Let's also not forget that a significant % of this flight are likely to be business travelers where somebody else, namely the employer, is paying for the ticket. IDB is cash compensation and it either belongs to the employer or its taxable income to the pax who didn't pay for the ticket.
VDB coupons are also likely the same, but businesses and certainly US tax authorities, IRS, don't look at it that way (now).
Coupons/credits/certificates are just as valuable as cash for the regular flier. Certainly not worth anything to the guy who will never use them.
So, even on this point, OP has another false premise.