Objective assessment is always biased by personality issues, by company culture, by in house politics [brown nosing] and by herd mentality [hangar talk].
It's hard to get into people's minds, but the fact is that some people just don't like each other no matter how well they fly and no matter what professional demeanor and experience they demonstrate.
That's why the universal phenomena called "unions" exists at many companies: Collective bargaining for better pay and improved quality of life issues [rostering/hotels/vacation/pay/etc]; and a seniority system. . . so that all qualified pilots have equal opportunities for upgrades.
The seniority system itself is no guarantee for upgrade, its only a means of fair selection. Indeed, many F/Os don't make it into the left seat, some are permanent F/Os. Irrespective of seniority and union protections, some Captains and First Officers are turfed because they can't pass 2nd sim or line checks.
In many countries, the seniority system [by date of hire] is the accepted industry standard for airline pilots.
