He wasn't good enough to be a starting CB, LB or DM for United. Would have provided good cover for all them positions but just because he's better than our current options, doesn't mean he's good enough. I do agree that he's a better CB than Rojo, Jones and Lindelof but if we have serious ambitions of winning the league/competing in the CL he would have had to leave eventually.
Rojo got a lot of praise on this forum and was considered our best CB for a period of time. Blind too, but towards the end people were happy to see him leave as they knew we needed better. However, the current dross playing at the minute bar Smalling are not better.