I'm sorry but I beg to differ. Yes he might be the sharpest minds around but there are not many teams who can afford a player who can bat no more than 2 overs and isn't a 100% behind the wickets. It's the plethora of all-round talent that CSK have had is what has kept MSD still into the mix. Having the guys like Jadeja, Kedar, Raina, Moeen, Bravo etc. over the years helped the team's balance and meant that they had plenty of batting depth to be able to accommodate Dhoni just as a specialist keeper while his batting (if it came good) being a bonus. That wouldn't work for say teams like RR, RCB or GT so yeah team balance has lot to do with his long career. The credit here actually goes to CSK's management who have managed to bag the marquee players around Dhoni so he can forget about his batting prowess and be a full time player-mentor of the team.