I take your point about the teams already in the senior league.
Even if I accepted the point about bringing through better youngsters, which I don't, I would still argue that it is a foolish trade off to make. No football fan is really interested in some abstract idea of aggregate, or overall, quality of young players in Scotland. Football fans want to know that when they turn up to watch a match that they can see their team win. If we keep structuring football in favour of big clubs then it will continue to decline as a contest. If it continues to decline as a contest then fans will continue to abandon the game.