Within a system were all employees have to regularly pass performance checks or get fired how else do you run a system that gives recognition for long service?
You would still get benefits for long service, leave, rosters etc, that would be something to consider when changing companies, the rest of the world works perfectly well without seniority, (a doctor doesn't become an orderly at a new company and likewise a bank manager doesn't start as a teller) simply put lifestyle and pay would be the driving factors, if either/or didn't stand up to the test and that would change over a lifetime (young, old, kids, divorce, marriage etc.) then one would move on.
The point being companies wouldn't have people handcuffed to a seniority system unable to leave for fear of battling it out at the bottom of a new company as the most junior pilot when you might have 5, 10, 15+ thousand hours.
It's steadily getting worse at the bottom, the problem is it's creeping higher and higher and the companies are realising that very few senior crew will leave as they is no way they can afford to start again.
Think about it, would you leave if the company cut your pay by 20% tomorrow and if so where would you go? Apparently the trend is it's a 100% pay cut to P2F at the bottom of a new company.