The company could base promotion on who worked the most g days, or who participated in a flash mob, or who signed onto yammer, or simply sell it to the highest bidder.
The seniority system is far from perfect, but it really is the only fair, transparent system we have.
That's applying the way people behave today as a result of being in a seniority system to being out of it.
Why do banks, law firms, hospitals not work this way? Because they would be absolute crap and have the dumbest most useless employees in short time, losing money, losing cases, and killing people.
Seniority causes people to try and buck the system and airlines profit from it.