Seniority does seem to have a bit to answer for. If it didn't exist it would be frustrating for a while as your company hired in new Captains over the top of you but after twenty years or so enough old boys would have fallen off the top of the tree to get you into the left seat. Also, if you were a sharp operator and upgrades occurred based on merit you wouldn't have to wait the full twenty years. Also, if your operator wasn't creating sustainable rosters you could simply walk down the road to the competition without taking a 100k pay cut.
Remind me again why we have seniority?