The biggest mistake they made was bringing the wrong set of people to handle the club from Ed Woodwards down to David Moyes, those were wrong choices and I believe Ferguson somehow played a role in their down fall cause he approved the selection of a wrong coach to supersed him cause he's his country man. Another big mistake Manchester United made was sacking Coaches like Jose Mourinho and Ole Gunner Solkjer if only they gave them enough time like Arsenal has given Arteta.
Well they'll continue to be left out of the top 6 if they fail to do the right things, good for them that they've employed the right people at the board level, what's remaining is helping their coach get his right players to carry out his instructions properly then they'll gradually begin to perform well again.
I don't understand why Manchester United made such a poor decision to entrust the club's affairs to the wrong set of people. While you believe Alex Ferguson might have played a role, I don't entirely agree. A retiring coach making suggestions for his replacement doesn't mean the club should have blindly followed his advice.
The club needed proper restructuring and with Ruben Amorim as head coach, Jason Wilcox as sporting director, and Christopher Vivell as director of recruitment, there may be positive changes and smarter signing of quality players that will align with Ruben Amorim's vision, making the club competitive again.