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 know why Manchester United didn't decide to start fresh, instead bringing the wrong set of management always. With the current structure, I don't think any manager will be able to implement their tactical plan effectively or achieve the desired results due to the deep rooted management issues engulfing the club. It will be difficult for Manchester United to compete for the Premier League title or even secure a top four finish, the club needs a clear vision, and a smart transfer policy if they truly hope for a successful rebuild under Ruben Amorim.