Originally Posted by Steve M

... true, but domestic F seats aren't going out non-empty (before upgrades) for the most part just because demand is up. A huge factor that wasn't always the case is things like TOD cash upgrades. It used to be that the majority of F seats would go to elite upgrades, with the understanding that there always was the possibility that there would be fewer or no upgrades on a particular flight if there happened to be high demand for people paying full boat for an F ticket.