To the first part, I said before that that particular summer he was coming off a literally historic season. There weren't 5 strikers better than him that year. That was his peak and it was unarguably world class. 36 goals in Serie A is literally unmatched.
I wouldn't say that by signing Higgers we were neglecting the midfield either. The Juve Team which got to Cardiff was the best we've been post-Calciopoli. Our midfield was world class, just like the rest of the team. Our lack of sound long-term planning would eventually bring us down, but tbh it was the summer we signed Douglas, Can and Ronaldo that really fucked us. Pjanic and Khedira as the 2 in a 4231 was top drawer.
Both the Higuain and Ronaldo signings came from an obsession to win the CL. Our squad was almost complete and was missing only a deadly finisher. That's why we splashed so hard - throwing our transfer strategy of bargains and opportunities out of the window btw- on Higuain. People were saying at the time it was a mistake because of his record in finals, and perhaps in the end they were right. So we went for Ronaldo to bring us the big ears. Didn't work either.
Point is the management identified ST as the only position in our squad which could be meaningfully improved. And they bought a top guy at the peak of his career.
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And btw this was while we already had Mandzukic. Allegri gets huge credit for the tactics of that team, because Mario on the left was the key to the whole thing.
There were 5 better strikers than him that year, as any other year. I explained his record, he stopped assisting at all that year compared to previous ones, and he was non existent in other competitions (only 2 goals).
Our midfield was Pjanic and Khedira, we didn't have a WC mezzala or AM since Pogba left to this day. We had to create a makeshift 4231, then 3421, to make it work. We played with 2 mids and had like 5 of them who weren't good enough on the bench, and we played with 3 strikers and had Pjaca as cover. The formation was something Allegri came up with on the fly, and basically one injury would have screwed up the whole team.
I wouldn't say our squad was complete since we missed some parts. AS was without alternative for years. When Dani Alves left we had De Sciglio replace him and Licht was in his last year, way past his prime. Our wings weren't elite and our final 3rd play from midfield wasn't there either. Also the squad was aging. A lot of areas to improve before splashing the whole budget on one player. Especially not on one that wasn't the absolute best out there.
Higuain might have been at his peak but that's also part of the problem, even back then you could see concerns that he won't last long. That's a 90M investment on a guy who had 2 years left in the tank. You're right we had Mandzukic, we had Dybala, Morata was leaving and needed to be replaced, but we definitely didn't need 90M to do that when we could have signed a mezzala and/or another winger in addition to a less expensive CF