Where Did It All Go Wrong For Manchester United?

Since Alex Ferguson retirement in 2013 one of the most trophy ladened English team has been reduced to would I call it nothing! Manchester United has been one of the most prolific English teams in the history of football, having 20 Premier league titles though out of those 20, Ferguson won 13. Ferguson won 38 trophies for the English team in his 26years at the club. So does this mean without Ferguson there would be no such thing as Manchester United?
David Moyes failure to carry the team to win any major trophy after succeding Ferguson seemed to be just the team making a wrong choice of manger. Same happened to Louis Van Gaal the following year and now The Special One, Jose Mourinho is now faced with an uphill task to bring the team back to stardom. This now begs the question where did it go wrong?
Even after a recording signing of Paul Pogba of almost £100million

just earlier this week they were trashed by there rivals Chelsea an astonishing 4-0 and had lost a lot of important matches prior to that match.

Is it that Ferguson couldn't find a suitable successor?
Is it the almost new era of players?
Or, is it the owners of Manchester United itself?
All these questions need to be answered together with the major question of the day, WHERE DID IT ALL GO WRONG FOR OUR WONDERFUL TEAM?