I Agree with you PP,
the issue isn't with seniority but In my opinion it is closely linked. Seniority is involved in many issues but I think it's fair to say that very few people jump of a seniority list because they can't get the leave they wish (for being to junior) but there are plenty of examples where junior F/Os have left an airline to get a command elsewhere.
In reality a command selection board will not look at all applicants as they have plenty of suitable applicants within the most senior pilots.
The key word here is suitable which relates to the previous issue of "pass" level.
Let me illustrate my thoughts with a example:
An airline have a need to train captains and they have a long line of applicants that fullfill the requirments. further down the list is another applicant that is comes highly recommended by the trainers. Maybe he possess skills and willingness to after achieving command move on to line training or has previous experience of such work. With most seniority systems you couldn't promote this individual.
One might say that there will be other with those skills but I think you agree that as you look at higher positions in a company the individual makes more of a difference (not everybody is cut out to be head of training etc). It is hypothetical, I know but I just want to point towards what I see as a bit of a backside of the system
In order to advance in a company a commands may be needed and it is a bit of a bottle-neck with seniority system.
Here we go again....