There are those who have tried to stress the fact that promotion based on seniority is not the norm. You have to pass the test. The time when you are assessed is based upon length of service (seniority) and required qualifications. Some do not seem to have grasped this. Those qualifications will include such matters as total hours, good enough standards at bi-annual checks, soound attitude in crew/pax management issues, stable personality etc. The company may want a sign that you have an interest in remaining with the company after your promotion.
In any field there will always be those who are average (good enough), average +, very good and excellent. Just look at sport to see the comparisons. Not everybody wins the tennis tournaments or golf; (Kornokova never won any but made a fortune); not every footballer is a star; but then a team needs a good solid longterm consistant performer, rather than a flash in the pan prima doona.
Thus, it must be assumed that all pilots are competant as they pass their checks. You now have the recognised excellent pilot who is an F/O. Older than his years; will make an excellent captain. Do they deserve promotion earlier than the plodder who is dependable, longtern career minded with your company, steady eddy rather than Top Gun? IMHO, no. How is this decision arrived at? Seniority and satisfying the qualifications to deserve assessment. Still got to jump through the hoops.
But I agree with the attitude that seniority sucks when it is always used so the fat cats get fatter by always having the biggest piece of pie. Seniority usually brings higher salaries. That should be enough.
Spreading the argument slightly: it is archaic that larger a/c attract larger salaries. A jet a/c is a jet a/c. Within the same company salary should be by rank and length of service, not type a/c. The salaries come from the overall profits/income of the whole airline, not fleet by fleet. Guys flying short-haul are more productive in pax carried per month than long-haul guys. Guess what. Seniority allows the top dogs to bid on the highest salary fleets. Why? To me it has always seemd odd that the oldest pilots want to be on the most tiring of fleets. Generally (IMHO) longhaul is a younger guy's world. Could it be that the salary is higher and thus the final salary pension is also higher. Fat Cats again. You also get the nonsense that flight managers need to be training captains - another bit of B.S.- and then their salary has to be higher than T.C's because they are managers. Guess what; in some airlines seniority is used to decide who becomes a T.C. Even more B.S and nepotism at its worse.
There has to be a system to decide who, out of a pool of equally qualified people, gets the first chance to prove they are suitable for promotion. Once that choice has been made it is necessary for the candidate to prove they are capable. There merit enters the equation. Seems fair all round.