What would be wrong with promoting based upon merit, rather than purely who’s been there the longest?
Absolutely nothing wrong with the theory of merit, in the same way as the theory of Communism "From each according to his ability, to each according to his need (or needs)" from Marx's writings is so seductive. It and required about 100 million to die to demonstrate the practice was flawed from inception as it lacked an understanding of Human Nature.
"Merit" gives the boss almost unlimited power to play Machiavelli. The seniority system is just the least worst system to curb the excesses of the psychopathic employer.
Bottom line is people in positions of power, ultimately end up abusing it, often despite their own best intentions & beliefs. There needs to be a system of tension between the employer & employee to curb the worst of either sides nature. There is no perfect system, and the seniority system is just one method of maintaining that tension.
The system also benefits the employer as he always has the perpetual "promotion" carrot for those stuck at the bottom as a way to make the employee sticky &
SUPPRESS T&C "You give up something NOW to be rewarded LATER". No carrot = less suppression of delayed gratification.
If the demographics & airline expansions in Asia Pacific pan out the way it looks, a group seniority system is exactly what a smart long term thinker in an airline would plan. But alas management appears to have a "rip the guts out of it now" mentality. The phenomenal cost of delayed recruitment & training is truely eye-watering by sending people all around the world. The $$ numbers I have heard are enormous.