Further to what
@s4tch said, old Allegri made a very versatile team that didn't need to play at 100% to win in Italy, but at the same time could switch it up and go toe-to-toe with the best teams in Europe like Barcelona, Madrid, and Bayern. I remember going to Madrid where we came back from the first leg loss to lose to a dodgy penalty. We would morph into different formations to close games out and we played some great games the last time out.
This version of Allegri is something different. He has a defeatist attitude, he's frightened to experiment, he has no faith in the attackers, and despite being a monumentally successful manager, couldn't muster up anything against our biggest rivals in one of those games that could've swung the title in our favour. Its not like he's new to any of it, he's done it before and succeeded so he should do better. We should've 100% beat Sevilla and won the EL, our squad was far better, but as always we lost when we really should've won. Our loss to Ajax made him quit, but Ajax were phenomenal that season so I'm not hating him out of frustration.
I was glad to have him back for maybe two seasons, but not this one, and don't want him here next year. He does not have a solution for us, despite all this time and a bunch of questionable decisions on transfers and tactics. The longer he stays here, the more accustomed we become to settling for this mediocrity. We can't compete against teams that we should be competing against. The Serie A title seems to be changing hands a lot, so that tells me that there is no single dominant team so we can get back into the race, but it won't be with Allegri.