Well yes our record of centre-back buys has been terrible, but do we have the answer on our books? I know that 50% of it is making a game safe by scoring, but you have to be able to defend as well. Kompany can. Not sure on the other two.
I think all 4 ( Yes including Kolarov) can. But more importantly, How well we do defensively is not just a function of how good our CBs are, but rather how good we are as an overall squad defensively.
The weakest points of that overall analysis of being good defensively were Keeper, fullbacks and scoring.
I know its counter intuitive, but scoring a lot early demoralizes the opposition and makes defending them easier.
Possession also demoralizes the opposition coz they spend so much time on trying to just get some of the ball time and thus little at actually what to do with it.
Making saves. Keepers make defending look better with key saves that slurs the defense.
Counterintuitively , Kompany's greatest addition to our squad play last season when he returned was his offense and possession (especially once his confidence was fully back.)
Defensively, contrary to popular opinion, he was a lot like Otamendi. He did a lot of the same things Otamendi often got chastise for. Gave up some unnecessary fouls, often rabid dog ran out of the backline to attempt to kill an attack before it started, and also made some dangerous slides. All quite Ota like.
However, where he was really immense for us was in his ability to dominate opposition in their box on corners and Freekicks. And also help us in keeping possession.
So yes I agree Komps is extremely important, but not necessarily for his defensive skills.
P.S:This is not a suggestion that Komps is not good defensively, but rather a suggestion that we are solid at CB... We are just better with Kompany because he brings more than just defense to the table.