A lot of people are considering the terms "long serving" and "loyal" as interchangeable.
In most other responsible, or professional jobs one could not argue against the 20yr service person on the home straight being overlooked for promotion against a keener or more capeable colleague with 10 yrs. That is just real life.
Seniority systems discriminate against, for example those who have been made redundant, and who may have shown the utmost loyalty to their previous company.
I have never been convinced by the benefits even though it worked for me, as I was "lucky" enough to join my company ahead of a recruitment bulge (by a few days!).
With the state pension system now assuming workers will retire at 70, the raising of the retirement age is good news, and I think potential returnees should view the extended career as a bonus, or at least not expect sympathy from new f/os with an 8yr (and now increasing) wait ahead!