Maybe the EPL was outdated back then, too?
Like it or not, Ferguson was not as good tactically and not as obsessed with details as the new generation of top managers. Guardiola's Barca dominated United almost at will and so would the current top teams such as City, Liverpool or recently Arsenal. I mean, isn't recommending David Moyes as his successor enough evidence that he misjudged how the EPL would develop? As said, that takes nothing away from Ferguson's legacy or accomplishments - he stood as among his contemporaries and it is an amazing achievement that he remained at the top for so long. But it is not disrespectful to acknowledge that the football he played wouldn't be successful anymore. Eventually, Guardiola and Klopp will be experience the same. Everything comes to an end.
Two of the positive examples you named have played less than 350 minutes for you this season and we're in April. I mean, it's great that Mainoo and Garnacho are playing well for you but if you think that makes United a great environment for young players, what should I say to you? The record I posted and cases like Sancho in particular will have every young player you're interested in think twice before joining you.
But to be honest, I don't really care that much. My point was rather that the assessment of player performances is pretty off in here. I've seen countless times that a player had a good game, recycling possession well, etc., and was later on criticized for being a "nothing player" because he didn't play spectacularly enough, especially when his team lost. But that's how football is played these days. People get frustrated with Sancho because they want him to take on the fullback all the time but don't realize that the "always passing back" he does helps the team more than forcing his way into blind alleys. The days of Giggs, Nani and co. are gone, wingers play differently these days.