My experiences , First time connecting out of Detroit where I wasn't upgraded before I got to the airport, so maybe they follow the rules differently.

I never once heard a GA ever say if you are on the upgrade list & want upgraded don't board the plane!

I fly almost every other week,sometimes, several flights in one week, mostly all domestic with Delta. Today is the first day I ever heard that if you board you lose. I have lost out on being upgraded by one several times. I have also witnessed customers being moved up to first class after being seated , I have also been moved up to first class after being seated & have left my bags at my original seat.
I believe one of our benefits of sky priority boarding is so we have a place for our carry on bags so we don't have to check them. So asking us to board last or miss out on being upgraded isn't really a nice option. I would rather wait to get off the plane last then to have to check my bags.

Why not release the first class seat if the passenger hasn't shown up 20 minutes before the flight,I have heard announcements about taking a risk of losing your seat if you do not show up within 20 minutes before your scheduled flight.

Why leave the first class seat empty because a customer decided not to show up?

I also want to mention that I have been next on the upgrade list several times and have never been asked to board last while waiting for a first class passenger to show up. I have never heard any announcements about this either.