I think I finally get what you are getting at. Since the system is seniority based, are u worried that people run up and take your seat at the last minute? Let me quell your fears....
In my 11 years here, I have been bumped off flights I thought I was going to make. On flights that are tight (meaning securing a seat could go either way), I have *NEVER* had an agent take away my seat after it was assigned close to departure when someone senior runs up to board. It seems like it just does not happen. If the flight is mainly full and departs at 3pm, lets say they clear non revs around 245-250p, I have never had them come down and yank me.
Does that help at all?