I think you have it backwards. Skill/attitude in service is subjective, while seniority is objective.

But you are correct that seniority trumps all.

Seniority is a convenient way for US-based airlines to avoid innumerable lawsuits from unionized employees. If AA assigned its FAs to certain routes based on perceived skills in service or demeanor, the company would spend all of its time negotiating with APFA to settle disputes about why the company perceived Debbie to be a better FA than Helen, etc. Similarly, if AA started hiring/firing FAs based on "temper" or "demeanor" or "grooming" or other things that the Asian airlines (for example) might do on a regular basis, AA could face BIG problems in the form of very expensive litigation involving labor unions and employees claiming prejudice in favor of attractive, leggy blondes, etc., as opposed to the 70-year-old granny who has worked at the airline for 40 years. There are many anti-discrimination laws in force that labor unions could employ successfully.

So, to keep things simple, AA and many other domestic carriers decided that the only way to keep the lawsuits and union disputes to a minimum would be to use an objective factor, namely seniority in the FA ranks, for assigning flights. Is it an ideal system? Certainly not. Does it keep certain FAs on the payroll, flying high-profile routes, who should have retired years ago? Admittedly, yes. But the alternative would not be sustainable for AA in the current climate of political correctness and anti-discrimination in the USA.