I have been doing a bunch of DEN-ORD and back flights and trying to understand the upgrade game - for example
my last flight - ORD-DEN on friday night (last nonstop of the day) - typically there are many seats available
1. A week before, 10 BF seats open
2. 48 hours before - same ten open
3. As a 1K on the list, despite the 96 hours perk - there was no movement on the list until the day of the flight - I remained at 5th on the list all week (and everyone else stayed where they were also) - I didnt use a PP to get higher since I knew (based upon past experiences) that at best I would be 3rd-4th vs my 5th place.. and with 10 seats, I felt my chances were good
4. At around t-3 or 4, the top 3 cleared...so I was now second with 4 seats still open (I guess someone bought up to first in the last few hours)
5. At t<1, I got my seat and shortly after they then cleared the last few from the list.....

I think the issues with PP and CPUs are, I cant recall the last time, as a 1k, I was upgraded earlier than t-48, let alone at time of purchase (PP are always wait listed) let alone t-96