But
That is not an issue of seniority, that is an issue of Command Selection process. The seniority has nothing to do with the suitability, merely when you get your chance to be looked at. Your argument is down to selection standards and would be equally true in an airline without seniority.
We are all individuals working within a 'system'. You have to set the bar where you (the pilot management/ pilot training management) judge it needs to be set to ensure the continued safe operation of aircraft under your regulatory system, thus protecting the best interests of your airline business.
If you set the bar too low you get unsuitable candidates passing who could go on to make serious errors in this safety critical industry (nothing to say that the suitable candidates won't have a bad day though, just that you are balancing the risk with your assessment).
If you set the bar too high you will only get exceptional candidates in the left seat, but will get many failures from us mere mortals who just don't make the grade and potentially not fulfil your left seat requirements which would have an effect on the business. You would also have many disgruntled right seat occupants who would potentially not be adding as much as they could to the operation. You will also lose many of them to pastures new.
This is why most airlines recruit 'future captains', not F/Os. They try to pick suitable candidates for command, based on their selection process, to ensure that they get a good number who, with experience, have demonstrated an ability to progress to command.
Having said all that, the short term needs of the business can come into play and it would be rare to find an airline cancelling flights because they can't find pilots........
PP