Merit based system sounds nice, but what merit could a pilot achieve? On-time
performance? Fuel economy? Less diversions, less missed approaches than average?
Smooth landings?
These items would mainly be within the captains domain, so not likely to be much help to an FO relying on promotion outside a seniority system.
I agree, what surgeons, etc get up to in their profession is nothing to do with pilots. Pilots are all required to be up to standard, if they are not then they fail and are not eligible for promotion anyway. Being better than the one above you is no advantage since the one above is good enough, having met the standard. That only leaves seniority as a basis for promotion. Remember, to be considered for promotion one has to be eligible by meeting the criteria for: Licence, type rating, relevant experience, (relevant hours) and above all,
suitability. Failing a series of checks as an FO would suggest you are not yet ready, therefore you don't meet the suitability requirement.
Most companies do a pre command assessment, often a LOFT exercise in the SIM and after passing that then comes the command training. So, even though one might have the seniority number, there are still a considerable number of hoops to be jumped through before being promoted to captain, it is by no means an automatic step.