No captain is perfect. He is human. Their performance last year was abysmal indeed, but look how they bounced back this year. The CSK management could have replaced like him like what SRH did to Warner, but they chose to trust him and look what happened.
Check my post again. I was saying that he was one of the best five years back. But nowadays, his tactics are not that good. If he was that good, then why CSK was in such pathetic position during last year's IPL? And let's not forget the fact that Dhoni was directly responsible for some of CSK's losses last year. In many of the matches where CSK was chasing the targets, the combination of Dhoni and Kedar Jadhav slowed down the chase and made sure that CSK loses the match.