I don't agree with the premise of the thread, but I can understand why one would think so. I have now seen threads blaming the the management hierarchy, the fans, the players, and the manager or the current predicament. The only thing left is the weather! In reality, blame can be apportioned to all of these parties in some form of fashion, and it all combines to make the job of a manager extremely difficult. Manchester United in a fairly unique proposition in world football. It is a storied, high-pressure club with a very large base of fans with expectations and a poor, commercially-oriented football management hierarchy and a spiraling media frenzy in the most difficult and competitive league in the world. All of these things combine to make this a cauldron under which only special managers could succeed:
Club Hierarchy:
Much as been written about the Glazers, but some of the main challenges in this regard have to do with how commercially-oriented they've set up the club management hierarchy to be. This has manifested itself in many ways from extended, taxing, football-suboptimal preseason tours to forced, commercially-driven older player signings to restricting squad composition and carrying deadweight on expensive contracts that are difficult to shift. Ironically, though, because they do provide money from time to time, the club can bring in the requisite quality of players, so that the club can finish in the top 4 frequently enough that certain commercial objectives are met, but also the prospect of hope or progress lingers. It's never bad enough that fan standards are lowered to the point where they will accept a few years of mid-table mediocrity in order to facilitate a complete clear out and let a young squad of players coalesce and grow together. So you are left with this yo-yo phenomenon where everyone knows that the owners are the problem and present structural constraints for the manager (e.g. no backing for complete squad clear out to reset the tone), but there are enough moments of hope interspersed that one periodically begins to think, maybe this time is the breakthrough.
Fans:
It's not really the "fault" of fans, per se, but it does contribute to the coaching environment. People keep invoking De Zerbi, Ange, Emery etc., but those clubs simply do not have the same expectations as United. That level of pressure and expectation will do funny things to a manger and, whether they admit it or not, will drive some decision making for all but the strongest of managers with the most conviction in their ideas. At United, you've seen a phenomenon wherein managers go into job-save mode as opposed to "implementing long term vision" mode, and decisions start getting driven by desperation and short-termsim, which is not the way to build a long term project. This is, of course, not helped by the club hierarchy deficiencies and seeming unwillingness to back a manager through difficult bedding-in periods like Arsenal have done with Arteta, but Arteta would also have had a rough time with the fans at United as well. It's very tough to draw comparisons in world football, because there are very few clubs with as much fan pressure as United, and the Bayerns, Real Madrids and Barcelonas of the world play in leagues where Champions League football is all but guaranteed and bouncing back to contending for a league title is not as difficult a prospect.
Players:
A lot of the issues with the players, as a collective, are contextual. With all of the manager churn and incompetence in the hierarchy, you often end up with a bad mix of players that is not-optimized for any particular manger or play style. Also, again with the management hierarchy, you are ending up with players who have their bellies full on great contracts and simply cannot summon the hunger to go the extra mile. Even when you do have players with a winner's mentality coming it, it become a Sisyphean task to try to impose that throughout all of the club once you realize the extent of the rot. In no way am I absolving the players, but once malaise has set in, it takes a good bit to militate against the established MO. They can be better. They absolutely should be better. But with conditions such as they are, it takes an extra effort from the players to run abasing the grain and power through difficulty. If not, that extra bit will fall on the manager.
So then this leaves the manager. ETH's flaws and deficiencies have been expounded upon at great length, so no need to retread. I do not think he is a special manager and that is what it might take to extract any sort of pleasure out of the current predicament. Before, I have pointed to Arteta and Klopp as examples of an incrementalist approach, given that they are at big clubs competing in the same competitive environment with the Manchester City behemoth. ETH (or future managers) could show continued, incremental progress either in terms of results, cohesion of play style or squad recomposition to show that his project is worthwhile. Even though you might use Liverpool or Arsenal as comparisons, they are simply not the same proposition as United because of the interplay of some of the elements above and the manner in which expectations of management and fans have been calibrated for those clubs, creating different operating environments for the manager. Unfortunately, it may take a longer period of losing and embarrassment in order for a sufficiently big enough recalibration of standards to take hold such that lesser managers can extract some joy. Even with the Glazers as they presently are, the right manager could come along with the right mix of tactical acumen, comportment, man management skills, charisma and force of personality to start generating winning results, but it is a very narrow window and a testament to how dire the current confluence of circumstances is that it would require such a talent to win again.