Meritocracy is good where the company is fair in rewarding merit and not favoured group.
Seniority is good where it is at least nation-wide and one does not have to start over again in a new company due to faults of not his own.
Present system is neither and unfair as it does not recognize and value previous experience with other companies and treats pilots not like trained professionals that they are. Similar professionals (lawyers/doctors/dentists/dental hygienists/truck drivers...) often, if not always, get a raise when they join a new company, pilots on the other hand have to start at the bottom at a fraction of their salary and worth as unfortunately, operators take advantage of pilots' love of flying.
If you say that they don't have to leave, well sometimes they do if the company goes bankrupt and plus what is wrong with career advancement. It seems that unions are a double edge sword.
On a lighter note, perhaps when pilots join a new company at a fraction of their worth, they should only demonstrate and fly with that fraction of their experience-one gets what one pays for!!