Whenever a great manager leaves a club, the club would have to struggle for sometime before they can go back to being great as they were, Manchester United dominated the premiership league and they won so many trophies because of the coaching style of Sir Alex but other managers are not up to the standard of Sir Alex except Pep Guardiola. If he manages Manchester United, they will be dominating the premier League as Manchester city are doing currently because he knows how to train players to play in the way he wants them to play and has read and understood the game of football and how to play it. Other managers are yet to do this, also Manchester United has the money to spend in buying players that will suit Pep Guardiola playing style. Manchester United need a very professional coach to returning them to the top of the league.
Manchester United does have a long history with all its achievements, so far we have certainly not been able to find another coach who is as great and smart as Alex Ferguson in building Manchester United, but if you read the long story of Alex Ferguson journey at Manchester United, of course you will know that Alex Ferguson also ever failed and was even frustrated at that time, I don't think we can equate Alex Ferguson with Guardiola, because Alex Ferguson was a more complex coach without relying on financial strength to build the club at that time, while Guardiola of course probably wouldn't have been able to build Manchester City as great as it is today without the unlimited financial support from Sheikh Mansour.
Currently Manchester United is in a bad phase and maybe we think that if Guardiola will be able to help them rise again, I agree with your thinking a little, but I think football is a very complex sport and does not depend on one individual alone, both the coach and the players and of course there are factors that must be supported to make Manchester United can be successful again as before.
But despite all that, maybe Manchester United should start by firing ten hag at this time.