I understand the viewpoint that seniority is somehow 'unfair'.
However it is a safety valve against unscrupulous employers such as Ryanair. For example in a seniority based airline there is usually a step salary system ( increases the longer your in the company to a defined maximum after 15/20 years lets say). Thats proper order in most professional industries.
In ryanair you have 'contract' guys (not really) in the company 5 mins earning more than people who have been in Ryanair for 15 years ! Thats wrong.Another example - recently brookfield ( the 'contract provider') decided to increase the pay of captains by a token amount per hour.They stated that this increase is valid when your current contract expires! So basically you could have a new upgrade earning more than a fella here for years,because the fella here for years could have penned a contract 2 months ago for a few years.So he is frozen on lower rates compared to a new DEC or an FO who is lucky his contract is expiring around the time he upgrades !
Things like staff travel get better with experience/rank as well. In ryanair/loco = none.
Seniority works very well in tandem with a strong union - like BA,Virgin,Monarch etc. Even without a union in the company,seniority would ensure a rising defined career path getting better the more loyal you are.
For me it looks like a safety valve against pikeys like Ryanair who use the absence of a union and seniority to take the p*ss.