I disagree.
Seniority reflects the length of time served with one employer rather than within one industry. Obviously there are many examples where seniority is taken from one employer to another such as in management positions etc. However these tend to be structures where there is often a unique requirement for one individuals talents. Airline flying comprises a large number of similarly qualified individuals who basically fall into two camps, Captains and First Officers. The only differential within these two groups is time served with the employer. Promotion and opportunity is normally offered the closer each group gets to the top of the respective list. There may be situations where a specific requirement exists based on an individuals experience generally, demeanour and suitability that precludes seniority selection (training being one example). However in the normal course of events it seems fair and proper to reward those individuals who have served the longest time with that employer.
A bit like a taxi queue. If you have waited your turn you eventually get to the front. You would be very unhappy if somebody was then allowed to push in front of you on the basis they had waited a long time in another queue. In airlines it is often the case that those pilots who have served the longest are generally offered (with exceptions) the choice of new fleets, better trips, greater choice, and promotion from one rank to another where applicable. The fact that an individual who may have jumped from one employer to another over the years should join a company with an entrenched seniority that places them over an existing employee who has displayed perhaps decades of loyality would be totaly unreasonable.
I suppose if you are in the "unloyal" or "unlucky" camp that arguement may hold little moral sway, but there is no reasonable rationale for it.