Are you guys for real? What on earth makes you think that promotion to captain in a seniority based airline is based on time served only?
Firstly, you need a track record of meeting the required 'minimums' in terms of ability. This covers ALL command skills, not just technical but manage the operation including CRM, ability to make decisions, plan ahead, work methodically through problems etc etc etc. This is usually garnered through viewing your training file with records from previous LPC/OPC and Line Checks. Notice I say you only need to meet the 'minimum' grade for command suitability. This means that you have to reach a certain level of ability in all areas and sure, some guys are going to be better than others, just like in any walk of life, but you are assessed as being SUITABLE if you reach at least the minimum grade.
Secondly, you either enter a hold pool of Command Suitable F/Os if your airline likes to hold a pool ready for vacancies that occur, or you do some form of Command Selection Course which may mean a series of flights with a TRE or other suitably qualified trainer/assessor when you 'run the show' from the F/O seat and are assessed at how you perform on the line making all the operational decisions. Other companies may have a simulator based exercise where you are 'loaded up' with lots of issues and failures and your performance is assessed. Only then may you enter the 'hold pool'
Thirdly, a vacancy has to arise that needs filling and if this is any time after you entering a hold pool then you need to keep gaining 'Command Suitable' grades in any sims/ line checks that occur in the mean time.
Forthly, you must then pass the Command Course, and believe me, many fall at this hurdle.
Where does seniority come into this? Well, your 'chance' at entering the process is usually done in seniority order, i.e. the most senior F/O is looked at first, if not command suitable then the next most senior is looked at and so on. This means that if you have patiently waited your time in the right seat in a career airline, you don't get trumped by the Chief Pilot's son, Fleet Manager's daughter and every senior pilots' 'mates'.
Another pre-requisite in many career airlines is the bid system. If you don't bid for a command then you will be bypassed and a more junior pilot to you may get the command. This is particularly important when you look at fleet changes and the like.
So there you have it. Please don't interpret a seniority list as the sole 'unfair' criterion for deciding who gets a command first because it is NOT.
PP