I dont think you spend that amount of money on a player that you dont see as having at least the potential to becoming a first choice pick. Surely as a manager you want to be constantly improving the team, pushing the players you have to be better, not just buying someone and saying, OK, this guy is nowhere near as good as what I have but he might come in handy one day when someone is injured. You would buy a kid to play that role, and even then you would buy one you hoped would one day make it as a top player who would make it in your team.
Would just seem a complete waste of time to buy players you felt were second rate. It isnt how you survive at the top, thinking, right, we have the best team in the county now, so we might as well just leave it as it is and buy some really expensive but inferior players to wait in the wings. No, you are always striving to improve, whether that is for today or with an eye on tomorrow.
Whether we all agree certain players tick those boxes is another question. I am deeply ambivalent about Young, was not that keen when we signed him - precisely for these reasons, and because there were other positions that needed strengthening. I think he has actually been a damn sight better and more useful than I expected him to be. But I have no doubt Fergie believed he had it in him to eclipse what he already had. Otherwise I honestly dont think he would have bothered to bring him in.