Sometimes that's true, sometimes it is not. Look at Liverpool, were they awesome before Klopp? They were not, but he came in and made that team into one of the best teams in the world. But there are also a lot of bad examples as well, look at how Roma (even though they lost) became better with Mourinho but Spurs became worse with same Mourinho. It is not always the case of good manager neither, there is a fit problem as well, there are managers who could make a team much better and there are teams same manager could make it worse too.
I am sure even Klopp or Pep could take charge of some teams and make them bad because their tactic would not fit with that specific team. Juventus is good, they are not horrible, sure they are not winning as much as they hope to, but they are still not a bad team at all, I am not sure if changing the manager would work now, but if they can get a manager who is both good and also fits the team then of course they can get better.
I think that Allegri has not yet exhausted his resources at Juventus and it is too early to even try to talk about his retirement. It's just that the capabilities of any football coach have certain limits and it's impossible to constantly arrange miracles on a schedule
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
Also, a lot depends on the football club itself and its management, because not every club, especially a big club, agrees to give time to the coach to implement his plans, if something does not work out in the process yet. Also, clubs do not exist in an isolated space and force majeure circumstances may occur that can change everything for the worse. Let's remember Conte and his departure from Inter, which occurred due to the deterioration of the club's financial situation. In any case, although the role of the head coach in the club is a fundamental factor in the absolute majority of clubs, but also the head coach is usually the first candidate to leave if there is no result.