Long post alert! Decline is not something that you will see overnight, as these players we had were all top quality, and even if they lose a step they are still quite good. Decline comes from lack of planning, not addressing the teams needs, not giving the coach the option to play what hes best at. keeping players forever is never the answer, but also not replacing them properly will make you lose a step.
We hired Allegri in 2014. He was handed a team that was set up by Conte. The defense was the same for 3 years, the midfield as well. Then in 2013 they finally added that ace up front in Tevez. So now you have a great team in all areas and a coach that knows how to use it. Max always said his preferred formation was 4-3 and then something (whether its a 1-2 or 3, depends on tactics and personel), and he found a winning formula with that team because he had players that can execute this philosophy. Tight defense, technical and physical midfield, flair and quickness up front. A team that can defend deep, press high, play possession football, counter attack, just versatile, great and complete.
Then the 1st problems, summer 2015. On paper it looks like a great mercato and we got great results out of it, but with what i said above in mind, not so much. We lost Tevez, we lost Vidal, we lost Pirlo. Now they did a great job at replacing Pirlo by simply making Marchisio the DM and making a great signing in Khedira (the renewal later wasnt so much great), allowing Max to keep his 4-3 part of the team in tact. But upfront we did some mess, instead of replacing the players we lost with similar characteristics, or atleast what the coach needs. Mandzu was a great pick up as well, you replace some of that Vidal and Tevez pressing and grinta with him. But where is the AM instead of Vidal? Ok, you can say we got Cuads, so we could just play 433. But then we sold Coman, and never signed a starting LW. We signed Dybala for 40M, who is a SS (a right sided one, same side as Cuads), we spent 18M on another CF in Zaza. Then we lowkey did the same thing with Llorente as with Tevez, instead of selling to England for money we sent him for free to Spain because he was homesick (just a year later he did move to England and i dont think he played in Spain since, so much for that. Same as Tevez moving to China for ridiculous money, which should have been from us). This is all us losing money for no fucking reason, while not improving at all. This will sound controversial but we never should have gotten Dybala, we had a team built for 433 once Cuads was there, we just needed a LW. Alexis Sanchez is what we needed, perfect Tevez heir and partner for Mandzu, with Cuads in the other side. Or Di Maria. Or someone else, but in that mould. Now THAT is how you replace the atributes you lost, and give the coach the team to play what he wants. Instead we got what we got, forcing Max to adapt and go 352 (with that team, the only and best option). Not what he wanted, not what hes best at, but he made it work.
2016 we lose Pogba. Marchisio is pretty much done and we replace with Pjanic, which is good. We sign Barzagli replacement in Benatia, again great. Dani Alves is a fantastic pick up. But we never replaced Pogba, as we sign a CF with his money. And now Max cant even use his 4-3 part of his prefered formation anymore because he lacks a starting CM. So he again adapts, makes changes to the formation allowing him to field the best players. It becomes a 4-2-3-1, and he still makes it work again.
2017 we get D. Costa and Berna, on paper good pickups. But the reality shows that we have only 2 starting caliber CMs (one of which is slowly on decline), and 4 starting caliber attackers. So again we focus on what is not needed. Matuidi is a good signing in terms of quality, but in terms of profile we needed something else. Or another CM in addition to him. Dani Alves leaves, we get De Sciglio. Solid backup and replacement for on-his-last-legs-Licht, but we lost a starter who we didnt replace. Bonucci left and again no replacement (Benatia was there but he was supposed to succeed Barza, who was done also by then). And at this point the decline was in full swing, because even if we were good on paper and had a great coach who made it work, we kept neglecting what was needed to be addressed and what our coach excells at, so we were in deep shit even if results showed otherwise. Max fooled poor Ronaldo as well, guy thought we are a great team and that he will have a great time winning the league every year comfortably, and compete for CL. In reality it was a team that needed to be rebuilt, handed to a guy that doesent know how to rebuild it.