The entire planet functions perfectly well without it, and no the doctor who's the cheapest or the lawyer who's the cheapest doesn't get the job.
The big difference being in a given country you have thousands of hospitals, law firms employing small number of doctors, lawyers. So a doctor, lawyer would have plenty of options, and seniority would be meaningless. In the same given country you would have one or two legacy carriers employing thousands of pilots. So a pilot would have very limited options.
Seniority within the company would always be there like any other job, however you could give three months notice whenever you felt like it in a 30-40yr career, what a novel idea. You might even return to CX after having brought a family up elsewhere.
I don't understand the above comment! So you ok for a company to have a seniority system internally where upgrades, promotions are based on DOJ, but you happy for same company to employ a direct entry Captain and bump all the First Officers? Is that what you are saying?
Even now you can still give your three months notice, plenty of carriers in China, Middle East, India hiring direct entry Captains, what is stopping you? Oh I see you want your seniority number at CX secure for life as you test the waters elsewhere, because you are special!!!
Show me one industry where you have a small number of companies that employ a large number of employees, all with similar credentials, where their salaries have kept up to those in other professions. Factory workers? Bus drivers? What makes you think that because you are a pilot that without seniority the salary packages offered by other carriers will be better? There will always be someone willing to do it for less, that has already been proven. You think a a brand new 777 rated Captain will not come to CX to join as a direct entry Captain and fly for the same salary as an FO? You think CX is going to care that this individual has less than 500 PIC time on the 777?