It's quite simple what Allegri brings to the club. Juventus have always been focused on dominating Italy first, then Europe afterwards. Max is cynical and result-oriented. We all know this. He doesn't really care about how we play as long as we win. 4 domestic doubles in 4 seasons are astonishing results. Chip in 2 CL finals and you have on paper a golden era.
When you have alot, you want more, it's human nature. And I understand fans wanting more attractive football. I agree that we play frustrating football at times and we aren't always playing to our potential. But from a business and straight winning perspective, let's say Allegri gets out 75 % of the attacking potential in this team. Are you willing to risk the domestic dominance and long European runs by sacking him? I'm not so sure we'd perform better with a Zidane or Pep. Look at Bayern this year, like us they have by far the best squad in Bundesliga, but are 9 points off Dortmund. Look at PSG in CL last year or even this year. Napoli outplayed them badly at times. Zidane at Real was quite lucky against Bayern last season, they got outplayed and were saved by poor finishing.
The main reasons I respect his work as a coach is by looking at the players who left his team. ALL of them have regressed. Morata, Pogba, Bonucci and even Dani Alves. So you have a manager who players enjoy playing under, who is winning (just not the CL finals), who guarantees a large amount of success AND develops players. The decision is easy in my eyes, It's not worth risking at all.