It's one of the reasons but not the only one or the main one.
The main reason each and every one of the aforementioned players is still here is because the manager who was at the helm at the time chose to renew their contracts, in some cases this happened more than once, instead of looking for potential upgrades in the market. This is usually what happens when you, as a club, bet all your chips on one man, the manager, to get the job done.
United are generating shitloads of money, thus we pay our footballers accordingly. We could sign a player from, let's say, Watford, tomorrow and his weekly wages will be instantly increased in order to match the rest of the squad's salaries. When used correctly, this ability can give us the upper hand against all PL clubs minus City & Chelsea when we're going after a player (we did it with Shaw, for instance). That's how it is and it's really not a huge problem to offload someone when a manager decides that a certain player does not fit into his plans. The manager can call a player in his office and tell him that he doesn't believe in him and that it would be better for him to look for another club. Or he can run down his contract (for the money) by spending a couple of seasons on the bench while his value slowly but steadily decreases.
Take a look at how many players LvG showed the door to during his tenure: Welbeck, Hernandez, Rafael, Nani, RvP, Evans, Lindegaard, Kagawa, Buttner, Zaha, Cleverley, Keane etc. Then in came Mourinho who kicked out of the club the likes of Schneiderlin, Depay, Schweinsteiger, Rooney, Blind, Januzaj, McNair and some other academy players among the numerous ones who were given their debuts under LvG, etc.
Valencia, Young and Smalling were regular starters under all three managers in the post-Ferguson era. Jones has started in almost every game under Ole. Rojo was trusted by LvG and Mourinho used him a lot until he got injured. Fellaini has been a wild card for some time now when plan A failed (which it did, a lot). Seemingly Darmian is on his way out with rumours suggesting that an agreement with Juventus is already in place. The rest of the deadwood is still here because the managers kept them here. They are part of Moyes, LvG and Mourinho's failure at United.
