I flew DCA-DTW on Saturday in the first row of C+. After boarding was complete, the GA came on and asked the pax next to me if he wanted to upgrade to F. Since he was not traveling alone, he declined the upgrade. GA left the plane and never came back to ask anyone else, and there were 20+ people on the upgrade list. So the flight departed with one empty seat in F. Since I was sitting right there I almost said "I'll take it if he doesn't want it!' but kept quiet. Several others were above me on that list.