100% we spent fortunes on full backs and wingers since Fergie retired and apart from James and Wan Bissaka can you call any of them a success?
Di Maria, Depay, Shaw, & Darmian are the obvious ones.
I still maintain that at the time we signed him Utd were not just a title challenger but a European challenger year on year he signed summer 2011 after we'd won the league and got to a CL final. We needed a new left winger as Park wasn't really a guy you wanted starting every game and Giggs by then wasn't a winger. Young had done pretty well at Villa but he was a bit of a one trick pony we all know it cuts inside and slings a ball to the back post or tries a shot he was pretty good at it but it was predictable.
Young was available cheap at £18m because he'd run his contract down to it's final year and Fergie took a gamble on him which I don't think really paid off for an attacking player his goal/assist/chances created stats were cr*p which is why I think he ended up being used as a full back because he was disciplined.
When he was signed he just wasn't the player we needed and I think he just became part of the furniture and he wasn't really any bother to managers so they kept him around bit like Phil Jones who came at the same time.