It doesn't matter who they go for, the problems that stop them winning titles and cups on a regular basis come from way above the manager. Nothing will meaningfully change at United until the culture changes first.
Moyes was going to carry on Ferguson's legacy and was hand-picked by the man himself; Van Gaal was going to update the squad with young players and move on from the Fergie days; Mourinho was going to get rid of Van Gaal's slow possession-based style and install a winning mentality; Solskjaer was going to expunge Mourinho's toxicity and get them playing football "the United way" again; Rangnick was going bring cultured football to Old Trafford; Ten Hag was going to bring proper coaching back to United after years of bad tactics. This idea that one guy will simply fix everything (whether that's a manager or a player) and make everything alright again is exactly why they're lagging behind the other elite clubs.
The closest they got to being genuine title contenders again was the summer of 2021. They'd just finished 2nd on 74 points and they'd genuinely outfoxed us in a few derbies with that counter-attacking style Solskjaer was a fan of - plus they'd reached the Europa League final and were unlucky to lose it on penalties. They weren't the best around in terms of personnel and their form was a bit streaky but they had a system that (mostly) worked - a tight, compact defence, a decent midfield with one technical expert, then three fast, hard-working forwards to exploit the empty spaces. It's easy to forget that they went over a year without losing an away game in the league at one point.
Then they went and added Sancho to provide more trickery from out wide (he was a massive upgrade on James at the time) and pulled off a pretty big coup when they got Varane to shore them up in defence. Those two signings alone, on paper anyway, looked like they'd be worth another 5-10 points, which put them in the title race for the 21/22 season. But then they signed Ronaldo and it fucked everything up. That signing alone disrupted Fernandes' form, alienated Sancho as soon as he arrived, pushed Cavani out of the team (and then he got injured anyway), and pushed Rashford back out wide. On top of that it turned United from a team who were happy out of possession and generally kept a good shape into a side who were all about servicing and compensating one man who scored goals but contributed fuck all else.
"The man to fix everything" had gone from being Solskjaer to Ronaldo.
In the end the entire thing unraveled because Ronaldo was stood up front doing nothing while the rest of the team were carrying water just for him. The best thing about United under Solskjaer was that (for the most part) there was no one player who considered himself more important than anyone else. They had Fernandes and Cavani as leaders, if you will, but they kept their heads down and liked Soslkjaer's system. But then Ronaldo showed up, which totally changed Fernandes' attitude, and that effective little system they had of Wan-Bissaka and Shaw supporting Rashford, Fernandes, Greewood, and Cavani was completely derailed. United were put on the back foot as a team instantly because they suddenly became incapable of defending from the front.
Solskjaer was gone within six months of Ronaldo showing up.
This is United all over since Ferguson left. They get close enough to reaching their previous heights and then shoot themselves in the foot - Van Gaal got United back into the Champions League and won them the FA Cup, and everything was set for Pochettino to come in and build on what Van Gaal had put down. He even
met for dinner with Ferguson in May 2016. But they caved at the last minute and decided to go with Mourinho because we'd secured Pep Guardiola and United's executives clearly thought they could reignite their famous La Liga rivalry. Mourinho started off well enough but his short-termism and toxic nature infected the club and set them down a path that resulted in Solskjaer, Ten Hag, and whatever the fuck's going on now.
Now, where they are at present is hardly a terrible place. They've spent the last decade finishing 2nd-7th in the Premier League, qualifying for European competitions, and winning the odd cup. There are hundreds of clubs who would dream of that sort of record, and United fans should do well to note that - as City have experienced - it can get a whole lot worse. But they are so pained and so anguished by their current position that all optimism and reason and balance has gone out the window. Their thinking remains impulsive, their culture remains a dismal, desperate one, where if instant success isn't secured then all the toys go out the pram and they have to start again. There's no attempt to say "Well, we have a good platform to build from, it just needs tweaks here and there" because the culture has totally consumed everyone who comes into contact with them, to the point where only new owners and a complete refresh will see them back at the top table by 2030.
Should they get him, Potter will be introduced on a wave of optimism and vague suggestions that he's "like Ten Hag but more of a man manager who knows the English game". He'll last 2 to 3 years maximum, maybe finish 4th and win a trophy in his first season, then the usual bullshit will rear its head again - players not playing for him, expensive signings not producing, massive injury pile-ups, etc. They'll slide down the table to about 6th and hopelessly bow out of Europe prematurely and get rid of him once it becomes clear that the situation can't be saved. And then they'll look elsewhere for the next new thing to get excited about before the cycle starts all over again.