Allegri is a successful coach on the Serie A platform. He has won many titles with Juventus. And since Allegri left Juventus, we have seen the decline in Juventus' performance. We have seen the irregular and weak performance of AC Milan for the last few years. Despite having a fairly strong squad, their performance is weak. This season they have got a great coach like Allegri. Even then, if Milan cannot perform well, then it is definitely their failure. And if Milan's performance is weak this season, they should make big changes in the squad.
Soon we will talk about Allegri like we are talking about Jose Mourinho like whether Allegri management style is outdated and he will no longer be able to gain success. Allegri can help AC Milan to improve and likely finish this season in top four but higher than that, it seems impossible for Allegri and AC Milan in this season. AC Milan have their problems while Allegri has to fight so hard to achieve success in AC Milan.
Allegri is a successful coach, but he also coaches in his own way, and not everyone understands that. In fact, there are few players who understand his instructions, and he clashes with many because he can't manage them. I understand that a player is young and often can't follow the coach's words, but if they listen to him, you'll see that there will be a notable change in the team.
A successful coach must have things that make him differently so it's very natural to see a successful coach has his own favorite tactical philosophy, management approach, and ego to stick with approach that built up his career success and reputation. Very few coaches can change their tactical and management philosophy over time and usually they made such changes in early of their career. After gaining success, it's hard to see considerable changes from successful coaches but football changes and after 10 or 20 years, there is a new trend in football. Consequently coaches belong to a previous era mostly struggle to adapt to a new trend, and they have to step back to give new prime time to a new coach generation.