I think you are mistaken on this point. As a Chairman preferred flyer on US I can tell you that even if the entire first class cabin is empty there still have to be 'upgradable' seats before anyone is moved up. They don't just upgrade people because of empty seats.

There have been several occasions when the front is only half full and they still do not clear anyone on the upgrade waiting list. They are trying to sell the seats, which they are entitled to do. Usually if I don't get upgraded in advance I will end up getting called up to the podium before boarding commences.