Despite what he was saying on camera, i think that, deep down, he knew that his successor would have to overhaul the squad. In that sense, he chose continuity and someone who would play the long game instead of Mourinho's short-term impact.
Where he probably got it wrong was that he believed that the existing squad would be able to carry Moyes and help him in his first couple of seasons. Both RvP and Carrick had enjoyed their peak seasons in the last two years (RvP had the first one at Arsenal), but, going into their 30s, never replicated their form again. 2013 was also Vidic and Rio's swansong at top level football and, for different reasons each, Smalling and Jones proved not to be the answer. Rooney was also pretty much done. That's the axis of a team crumbling in front of your eyes.
Don't know if Mourinho could have kept that team going for another year or two. RvP's injury practically killed Moyes. The Dutchman's ability to interchange positions with Rooney and be a threat in both half-spaces was crucial for us (thus, Fergie's comment to the players that they should make the best of what he brings to the table). Without his intelligence and off the ball movement, we were stuck with a physically declined Rooney to lead the line, plus: a) a one-dimensional right flank and b) a left wing that had only one function, Evra on the underlap and Nani cutting inside. Give or take a few details... pretty much like now.
Another big thing, one that could be laid at Moyes' feet, was that Fergie partnered Carrick with Giggs/Scholes for a reason. He wanted a creative threat from central channels with Carrick acting as cover. Moyes' always favoured a more flat 442-ish/4231 system with all the creativity coming from the flanks (see first point). Mata, as good a player he was, could not solve these issues and offer variety in the attacking third. His role, either on the wing or central, is more defined and restricted. He couldn't interchange positions with the forward and he could cut inside, but he was never a threat from these half-space positions. He was a different kind of player. It was the first time we got excited for a big-money signing that could not solve our issues. A taste of things to come, one would say.
I also believe that Phelan and Maulesteen being dismissed by Moyes wasn't as much of a big deal as people want to it to be. Neither of them was self-luminous, the best thing on their managerial CVs was doing what Fergie told them. How much help was Phelan when Solskjaer lost the plot? I also agree with those suggesting that the initial thought was for Giggs to become SAF's assistant before taking over from him. We actually tried to follow that plan, LvG was open about it. Moyes was an upper midtable manager who walked into a team that needed great planning ahead and found himself in way over his head. This also rings a bell or two in United's post-Ferguson era.
Some things never change.