Maybe, but he isn´t, although he should play consistently right now to develop properly. The hard truth is that McNair is not ready to play as starting CB for a club as ambitous and reputable as United. Your defensive stability is right now the strongest part of your game and I don´t see you guys jeopardizing that by giving him the opportunity to grow and freedom to make mistakes. I would loan him to a team, where he would start and then evaluate his skills in one or two years.
Stones is way further in his development, very composed for a player his age and already a very wellrounded CB. If he would come to United right now, I would see him starting ahead of Blind alongside Smalling.
Blind might have the build up play to contribute a lot from the back but I´m still not sold about him being first choice vs. top opposition. There will be opponents, which won´t be stopped by the solid cover your compact midfield and defensively excellent partner provides. Teams who will target his less than ideal positional play and weakness in the air.
Jones is a player with an amazing attitude and lots of potential (especially as ball playing CB), which was sadly hindered by many injuries. I would say that he probably already lost two years of development by now, although I believe he could catch that up if he would finally get a proper run of games. As it stands now, his positional play is similar a worry as for Blind. He had a lot of excellent tackles, which brought him praise on here, but were infact recovery actions for his own mistakes.
Having said all that, I don´t think that Stones is the right target for United right now, especially for that price. He might be an overall upgrade to Blind, Jones and McNair right now, but I would rather see them going for a player with more experience and the ability to teach and lead this fairly young back line. I think that all of them would benefit for having such a player in the team.