Well I’m certainly not going to sit here and defend seniority, but there are pros and cons to everything. Having worked at two airlines without seniority - the downside was a system in which a significant element of the promotion process was being on good terms with the chief pilot, and being a yes man and not creating “issues” or being “awkward” (no guidance on what that might mean in practise!) This lead to less than ideal candidates being promoted.
In contrast, the current airline I work for has a very transparent command process which, yes, does of course partly depend on seniority, but has little to do with you being best mates with the chief pilot or head of training…
I think there’s pros and cons to both, and one makes one’s choices accordingly.