How about "People who bring a roller bag and a backpack" and their backpack goes under the seat 'in front' of me, forcing me to put my personal item 5 rows behind me as overhead bin space is already taken by roller bags. The one time not in F yet could be first off the plane and I have to sit and wait for a break to go upstream to fetch my bag.
I have 'in front' in quotes because my seat was 19A on 757. So I suppose ownership of that under seat space is debatable. I just understand the OP's frustration when people feel the need to take all their belongings on board and take up space when people who don't take as much on board get denied convenient space when the space is limited due to aircraft design.
I get it, first come first served, and you snooze you loose, luck of the draw et al, but it is a valid pet peeve.