What is success for Manchester United? It's an important question. Every manager would do better with a better structure. We'd also replace those who aren't right much more quickly and have a smoother transition in between, have our peaks last longer.
Have ANY of the managers we've had been good enough to be successful at United if all else was good? That depends on definition of success. If success is becoming a top club (solidly top 6 in Europe both in results and underlying data/performances), then yes, I would expect any manager to be able to get there with our financial strength. If the success is winning the premier league, then no. I don't think we've had a manager on par with Pep or Klopp, which is mainly due to I don't think one exists out there.
Can have a fluke year, but realistically, since Pep came in the league he's been that far ahead. Klopp is up there, and he managed to build a squad to compete for a few years and win it once, but that's the level. You need to get everything right and be a high 90 point team for multiple years, and even then they only won it once throughout a 5 year span where they were basically a top 3 club in the world. So realistic plans on "when can we win the league again" are usually tied to Pep and Klopp leaving.