Some legacy carriers have straight line date of joining seniority and stick rigidly to it, subject only to pilots meeting the required standard. Most first officers would endorse this as the fairest and best, because they know exactly where they stand.
Maybe ex-military jocks with heaps of prior jet time would not support that concept and would prefer some merit-based system. Merit-based systems have to be handled very carefully because they are open to corruption and brown-nosing if there are not suitable checks and balances to remove bias. I like a combination of merit-based and time-in-service, whereby 'points' are allocated for performance and years with the company. That weeds out those who sit back and do nothing other than meet the minimum standard. But that's my view.
Most low cost operators have rather loose systems a bit like you describe. Often driven by beancounters not wishing to allocate money to training.
Looking after 'old mates' is endemic in this industry, and with union power so weakened, you are on a hiding to nowhere trying to change it for the better.
As for the fairness of those hybrid seniority systems that are honoured, do research on these forums and you will find much debate.