Maybe I don't understand the new UDU policy, but I thought it meant that those with status would get upgraded if there were seats available up until the flight boarded?

If there are seats available and the aircraft is boarding, shouldn't the gate agents print up new boarding passes for elites (based on status) until all of the F seats are taken? What is the point of UDU if they don't fill up the seats, assuming there are people who would qualify for those seats?

I understand "holding" some seats on the chance a full fare paying passenger would run up to the gate 20 minutes before takeoff or if a crew member needed to be moved to another airport, but allowing seats to remain empty does not make sense, assuming there are status passengers on-board.