I think Sir Alex was also part of the problems that befell Manchester United when he resigned. News then had it that he was the man that handpicked his successor which was David Moyes. That was the genesis of Manchester United problems, an inexperienced coach handling a team like Manchester United at that time, this was a title winning team that was handed over to him.
I totally agree with you on this, even in history, it mostly doesn't end well when an outgoing person handpicks his successor. I once read an article about Ferguson picking Moyes. How could he pick an inexperienced coach to manage a team like that. Moyes has been coaching Everton for almost a decade if not more and ends up mid table, how on earth did he think such a man was suitable for the Manchester United job back then! So I agree with you when you said Ferguson was part of the problem.
What are we even saying here? OK let us agree that Sir Alex Ferguson was part of Manchester United problem for picking his successor after his departure. What happened after that? Was it still Alex Ferguson that still pick a coach for them until present day? How many years are we talking about here that is not enough for them to correct that mistake? Their present predicament has no correlation with what happened for more than how many years now. This argument could be a valid point if Moyes is still in charge of the club all this while. At this point, it's safe to say Manchester united is yet to identify their problem talk more of fixing the problem.