These situations are quite rare. I don't think paying IDB in cases like this would increase fares by six dollars per passenger. This may be the case in Europe as airlines are on the hook for mechanical delays which I don't favor.
The rules need to change so that IDB is payable to any passenger who is lawfully entitled to board the flight by: paying for ticket, checking in on time, being fit for travel and having the required documents. As long as the flights is not canceled the passenger should have been boarded somehow, perhaps by getting authorisation from a higher up at the airline. No payout for any other reason though; that's what insurance is for.