I understand all that positivity coming from you as a fan but perhaps you are thinking in a little too overly positive manner.
Smalling Evans Jones have done enough to prove themselves as decent defenders but nothing to suggest that they are World Class defenders and more importantly Defensive Leader. That is the reason why majority (if not you) were asking for the likes of Hummels, Mangala etc after losing Vidic and Ferdinand. Rojo mainly played as CB for Sporting but its his performances as LB for Argentina that attracted interest, same goes for Blind who was brilliant as LWB.
Now, while playing three at the back and with not so brilliant defenders, United needed a strong holding midfielder and I do not see that in Blind and perhaps even Van Gaal who always preferred and reiterated the importance of Nigel de Jong. Herrera and Carrick may form a decent creative partnership but will not help the defense much. Hence everyone was demanding a complete midfielder in Arturo Vidal.
Not saying that its all shambles for United at the back and midfield but compare it with the likes of those challenging United for top 4, you get these are the departments that will again be the reason if United fail to qualify for CL again.