Seniority is a bit of a sugar pill really.
99% of pilots in an airline are just your average, capable, unknown to management operators who would pretty much get a crack at promotion based on time of service regardless of a seniority system.
The other 1% still manage to bypass seniority and get the “jobs for the boys” in the form of manager, assistant manager, technical expert, etc roles.
For the airline, seniority ensures a stable senior workforce that is pretty much going to do as they are told.
If it all becomes too difficult for the airline to fulfill their obligations to seniority, they simply start or buy a splinter group to get around it.
The answers (perhaps):
No seniority, but a powerful industry body like the AMA, with some control over the supply of pilots (limiting license numbers)
Or;
An industry wide seniority - you get a number when you get your CPL or ATPL.