I'm not overly concerned about United in the medium-term. We can put out an U23's side that looks like this
de Gea (22)
Fabio (23)
Rafael (23)
Keane (20)
Smalling (23)
Jones (21)
Zaha (20)
Powell (19)
Lingard (20)
Januzaj (18)
Welbeck (22)
We've got players ages 24/25 such as Cleverly, Evans, Fellaini, Kagawa, Hernandez (I exclude Anderson from that list

)
Moyes is right in the fact that the squad lacks a number of real world class players, but at the moment for whatever reason, they are either unavailable, or do not want to join the club.
Vidic, Rooney and Van Persie fit that bill at the moment. Nani potentially could have, but he's just the most frustrating player. Carrick is not world class, but has always done an excllent job for us. People question his inability to turn up for the big games, but he hasn't had a quality central midfield partner for few seasons now.
It might well be a trophyless few seasons, but it will be more of a dip than an unstoppable decline.