there's no seniority protecting promotion, then people will stop sharing the good information, and safety is degraded.
How do doctors or any other profession survive without seniority then?
You can't apply the way people behave as a result of living in a seniority system to a system without it, you simply have to look at other professions.
A doctor goes to medical school and one day is a senior doctor/surgeon/head of dept/chief of staff or whatever and he may have stayed local or moved from hospital to hospital or wherever depending on his own circumstances, he may have simply choose to remain in small town practice.
A lawyer goes to law school and one day is a senior lawyer/partner/barrister/judge or whatever and he may have stayed local or moved from firm to firm or wherever depending on his own circumstances.
Pilot, flight school and one day senior captain/fleet manager/DFO or F/O whatever and he may have stayed local or moved from airline to airline or wherever depending on his own circumstances.
Seniority (experience not # on a list) would exist like it does in all professions but you're right a **** doctor might never be chief of surgery no matter how much arse he was licking or however long he stays at said hospital.
So except for pilots those profession pay to keep you, allowing experience to walk out the door with a month notice isn't good for business.
Apply a seniority system to any of the above careers and watch the degradation begin as they sell their soul to join the bottom of the list.
Change Captain to Senior pilot and F/O to pilot, saying you'll never become a senior pilot without a seniority system simply isn't true.