For reasons I laid out here
But to be more specific, the reason I think he will fail is becaue like I said he doesn't have the gravitas, but also doesn't have the personality, to actually overthrow things and cut the dross.
His character in terms of player selection is heavily tilted towards fear rather than reward. He will always choose a guaranteed 7/10 performance now over a potential 9/10 player by the end of the season/ a year from now.
So where a Peter Bosz in his one season at Ajax brutally told players like Bazoer and El Ghazi that they weren't good enough (even though Bazoer represented a lot of value before that point), and was willing to put massive faith in 18 year old Dolberg (despite having brought in Traore on loan from Chelsea for the striker position), 19 year old Onana (depsite having brought in Tim Krul on loan), and 17 year old De Ligt. This lead to a difficult first half of the season, which ended up costing us the league by like 2 points in the end, despite a fantastic second half of the season and a Europa League final reached with this incredibly young core, which then developed (also thanks to Keizer taking the crazy risk of putting Frenkie de Jong into the first team at centreback the months after Bosz' departure) into the core that Ten Hag reached the CL semifinal on (together with the purchases of Blind/Tadic in his first summer at the club), and which resulted in Ajax' biggest ever financial windfall (the CL money + the €150m De Ligt+De Jong money). Also Vd Beek got starts towards the end of that season under Bosz, including in pivotal knockout EL games
Ten Hag on the other hand won't cut out any dross (not even from the bench) and won't risk any youngsters in that same way, and so it's impossible to build towards a future in which you don't continue to rely on this same set of players, and as a result you end up desperately holding onto them when offers come in for them, until eventually you're stuck with players whose value has diminished, whose contracts are running out, and a bunch of youth players whose development has stalled/hasn't been given a chance, or who have had to leave the club and immediately shine elsewhere (see Noa Lang immediately being player of the season (basically) in Belgium despite not even making our bench under Ten Hag, and Botman winning the league in France with Lille immediately after being sold, though in his case we had Martinez and Blind for his position so I get selling him for that money at the time)
And United need someone with the sway or at least character to ruthlessly cut through the rot that's been running through the squad this past years, but Ten Hag, being so overly cautious in this regard, would never be the man to do this even if he had that sort of influence/freedom within the club