Rot? No, but stagnation? Yes. Stagnation and a lack of attention to the signs of steady decline. But then again, when you're winning it's much easier to sweep any problems under the rug. It's the course of human nature and, besides, everyone despises the perennial moaner during the good times. But, even then, it was quite easy to tell that Ferguson's last United side (the one that won our last two PL titles) was rather lacklustre compared with the treble-winning one, with the '06-'09 juggernaut or even with the one that ended our draught by delivering the first two PL titles in the early nineties.
In hindsight, of course, the 2011 & 2013 league titles look quite similar to the last two titles Liverpool won (1988-1990). It wasn't the overall quality that made the difference but the habit of winning which was embedded into the minds of the manager and certain players who, in their turn, transmitted it to the others. In its full force, it created the famous fear factor. The sad fact is that many of those players were past their prime and/or reaching the sunset of their careers (Ferdinand, Vidic, Evra, van der Sar, Neville, Giggs, Scholes, Fletcher, Park etc.). Ronaldo's departure was the cherry on the top of the cake but, at the time it occurred, Fergie believed that we would miss his numbers more than we would miss his leadership. And, truth be told, the old gaffer always knew how to make the numbers add up.
This doesn't mean that there wasn't a plan. There was but it was one that, ultimately, failed miserably. We can now say with certainty that De Gea is probably the only one who provided us with an almost seamless transition in a position on the pitch from one world class player to another. In the centre-half position, too much faith was put on Smalling, Evans and Jones to replace Rio and Vida (Ferguson believed at one point that Jones had the potential to become one of the best English players). In the full-back position, too much faith was put on the twins to stay fit too. In the midfield, Cleverley was supposed to develop into something more than a midtable CM and Anderson was also supposed to mature a bit and eventually replace what he had inside his head with some brains. Unfortunately for us, none of these things happened. In the winger role, there were hopes for Nani (25, then) to pick up the baton from Ronaldo and take on a more leading role but his inconsistency plagues him until now and has hindered his career overall. On the other side, Kagawa never managed to replace what Park brought to the table for us and we also failed to get from Kagawa the qualities he had shown at Dortmund. Up front, Welbeck never looked like a natural goal scorer. On the top of all that, RvP failed to replicate his magnificent12/13 season. And the only player who was assessed 100% correctly by Ferguson who wanted to ship him off, Rooney, was given a hegemonical new contract.
But Ferguson always knew how to take the best out of his go-to men on the pitch while retaining a strict discipline at the same time. Rio and Vida's leadership skills were kept for the difficult matches mostly. Evra was playing, more or less, as a wing-back with the LCB or a midfielder keeping an eye on his spaces during the defensive transition. Giggs/Scholes were rotated smoothly in order for their experience at setting the tempo to not miss from our team. And I believe that both RvP and Carrick were allowed to follow personal training (meaning less intense) in order to stay fit. The big names were being taken good care of and the job was being done. And that's probably why Ferguson advised Moyes to keep the backroom staff intact but Moyes had other ideas.
Since then our biggest mistake has been the fact that we fail to realize just what we're missing so dearly. We sign players whose best footballing years are ahead of them/are in their prime (Fellaini, Mata, Rojo, Blind, Herrera, Darmian, Schneiderlin, Mkhitaryan, Lindelof, Pogba, Di Maria, Matic, Lukaku) and we also buy/promote youngsters (Januzaj, Depay, Martial, Lingard, Rashford, Shaw, Bailly) and we burden them we responsibilities that they never had before. We ask of them to come here and lead the pack to glory. And when they fail, we come on this forum and we wonder if there's more deadwood to ship off or if the young prospects are as good as we think. Out of this squad only DdG, Matic and Mata seem able to lead the team. Pogba is getting there but he's not quite there yet. With Matic/Mata Mourinho was spot on: to buy the former and to keep the second in the team. We need to see this and start addressing it. Otherwise, when we reach midpoint through every season, we'll keep finding ourselves resorting to the Valencias, the Youngs, the Smalling/Jones/Rojos to steer the ship. And we'll keep rendering these useful but not great players as useless every time we fall out of the title race.
You don't need anyone to tell you that what we lack is proven quality, the 'been there, done that' kind. Fergie had Robson and then he brought Ince to the side. When the latter became a mainstay in the midfield, we signed one of the biggest talents in Keane. And it was next to Keane that Scholes flourished as a footballer. Then Carrick was signed to do the job next to Scholesy. It doesn't matter whether you buy them or you promote them from the academy because true talent can be found everywhere. What matters is to have at least one player in each line to whom the rest will look up to (and of course able to perform at the highest level). And you can't always expect to create these players for your side out of promising youngsters/great players but with supporting roles in their previous clubs/past it greats out of which you hope squeeze a few good years. Sometimes you just have to keep knocking at the door until you get what you want.