WHile agreeing that assessing the skills of an individual is never easy....
the people who design the aircraft you fly are given relative authority over design decisions on the basis of their fitness to perform the task, assessed in that fashion; and
the people who approve the certification are generally selected for technical merit and not on pure calendar experience.
I would rather the better person do the job, not the one who had simply been around longest. Just because it is hard to assess people is no excuse to not try to do so.