First I need to know if you are talking about the accomplished versions of these managers, if yes then we need to keep in mind that United have done large squad overhauls for nearly all managers with Moyes being the exceptions due to his own way of doing things. So in United's world no player is actually safe and all managers have been given a lot of money to build their "own" team and there is no scenario where these managers wouldn't be given even more power.In a fantasy universe we have hired Sir Alex, pep or Johan cruyff. Two years later rashford is running like he is pulling an anchor. Hojlund is getting little service. Players aren’t doing extra work. Players aren’t trying to win a 40/60 - results are shit. Do we sack them? Do we let players countine being arsehole??
Also one of the issue which is kind of trivialized by the framing of your questions is that Sir Alex, Guardiola and Cruyff are/were infinitely better managers than anything we have had since 2013. It's like comparing prime Ronaldo with a washed up Jermaine Defoe.