Every airline that I have worked for (and it's been a few, see above) has taken on Direct Entry Captains. That has been airlines with seniority lists and airlines without seniority lists, When there have not been enough suitable (and that means minimum qualification and grades criteria), or willing (!!), pilots to promote then captains have been recruited from outside. Every single one of them has stopped recruiting Direct Entry Captains when there have been suitable pilots from within to promote.
So the argument that recruiting Direct Entry Captains 'deprives' the existing pilots of promotion has just not been true in every airline where I have worked. Regardless of them being 'seniority list' or 'non seniority list' airlines.
Airlines understandably want to promote from within first as those pilots are 'known quantities' and that has been the strong feeling in all those airlines. A proper process and transparency are what is needed for promotion and that has absolutely nothing to do with a "place on a list". Having a 'mate' in the sim putting you through the grades with a nudge and a wink is just as possible in a seniority list airline as it is in a non seniority list airline, just as the chance of some @rse with a grudge could block your chances in both (neither type of airline has any particular monopoly on those...).
So could those banging on about that non-starter 'issue' just look at reality and not your emotions.
And getting back to my comment about 'willing': I have know pilots in seniority list airlines, who were totally capable of promotion, not being willing to be promoted because their position on that seniority list enables them to have the sort of lifestyle that would be impossible for them if they took promotion. I haven't heard any of the seniority list proponents mention that!
However, I don't see this getting anywhere, both sides have already 'taken to the trenches' and many, many, many pilots' jobs will have to be lost before a victor emerges. (And I, personally, don't see the eventual victor as being the one using old fashioned methods in this 'battle'.)