The Structure/Board
Problem
The Glazers, who have leveraged large debts on the club, have not provided the proper vision and direction for it to succeed.
Yes, many trophies have been won under them. But largely in spite of them, and not because of them. The brilliance of Sir Alex has been a major factor, including the FA Cup under van Gaal, and League Cup and Europa League under Mourinho.
However, there has been a tendency to focus more on United's brand, and immense revenue generating capacity, over winning.
The tenure of Ed Woodward, the club's Executive Vice-President, also has come under fire for not providing a proper vision, and not handling transfers well enough.
Solution
Change the board.
Glazers must sell.
These may seem simplistic, but they would solve the issue.
Nonetheless, the Glazers selling their interest to another may cause additional problems. Who is to say the next owners would be better? They could be like Mike Ashley (not investing bare the minimum), Kroenke (similar model to the Glazers, but focusing too on revenues and not competition), or as a best case FSG (increasing Anfield, hiring Klopp, and enabling top spending). Though even "good" owners like ENIC at Tottenham have come under flak, namely for the Tottenham Hotspur Stadium debacle.
If the board is to stay, and the owners, then they need to be more open with their vision, and state clearly how they wish to invest in the club or ensure the club remains successful.
The structure also must change, since there needs to be a greater focus on footballing success.
Arsenal is often seen as being in a similar boat to United since both clubs are seeking to transition from long-standing managers. However, Wenger at the end of this tenure was failing. Sir Alex left at the top, literally, since United were champions at that stage. Arsenal thus have the motivation to ensure they get competitive, and there are more things needing fixing, and more impetus to fix them. Appointing Emery (a known tactical/meticulous manager), Sven Mislintat (head of recruitment formerly at Borussia Dortmund), and Raul Sanhelli (head of football relations formerly at Barca), are all key components in the defined "catalyst for change").
Arsenal had a powerful manager in Wenger, who controlled all facets of the club's footballing operation. But this has been split and defused, and exact components have been brought in to correct the perceived errors. Emery is looking to get Arsenal on track, via better tactical analysis and defensive structure. And with Wenger in charge of scouting and football relations, getting in two world-class replacements certainly plugs the gap.
United needs a similar catalyst for change since the club needs renewal like Arsenal did. Whether this renewal will take shape is moot. But then at the least, the club has acknowledged that it has structural issues, and has sought greatly to correct them. United now needs to do the same.