SOP = "Standard Operating Procedure"
I think we can all agree that no one, once upgraded, should be downgraded just because someone higher in the pecking order appeared. However, this is part of why relatively few upgrades clear when the window opens.
A year or two ago I was flying PHL-LAX-LAS on a paid F ticket. My flight to LAX got in early and I was able to get to the gate on an earlier connection than the one I was ticketed on, about 25 minutes before flight time. LAX-LAS is a short flight and all elites are eligible for complimentary upgrades. However they still had F seats, well, at least one. So in that case they may have been holding upgrades even closer than 25 minutes to departure.