There remains the fact that holding CPU's to the gate is an advantage to many. If you are a 1K with a flexible ticket or making use of SDC, there will be space in F and when you change/SDC to the new flight, you will wind up at or near the top of the list. If the F seats are largely gone down as far as Silver at T-24, your change/SDC is into a seat in the back.
Conversely, CPU's are freebies. Holding them to the gate doesn't harm anyone as them what get the UG get to sit up front and that doesn't happen until you actually board.