#4 Negative: There is no suitable replacement out there
United have to think about upgrading their manager rather than just replacing him. Any desperate attempts to get quick results may again backfire, as with a new coach comes a new list of players to be bought, chucking off all hard work done in the past 5 years.
United fans would definitely welcome Zidane with open arms. But even Zidane doesn't have the experience of building a team from scratch or taking out a team from a crisis situation. At Real Madrid, he had the best players and the best resources to achieve what he achieved.
All worthy managers like Klopp, Pochettino, and Ancelotti are already having long-term contracts with their respective teams, and it looks highly unlikely United will get a manager of that calibre.