Cricket particularly leagues such as IPL is not only about winning/performing. It is also about legends coming together and adding glamour, drawing audience. It is about respecting the legends (That is why even guys like Dravid, Laxman, Ganguly, Kumble, etc. also played when they were well past their prime, age, what not...)
Most teams in this league are not even having a proper wicket keeper (Still Dhoni has such reflexes) and so many teams you feel lack so many kind of players (After all these are not full international teams, full-fledged teams - It is also about giving opportunities to youngsters, serving financial needs to seasoned cricketers who can't make it big like Bhatia, etc).
So you can't call for retirement of Dhoni based on his form, his captaincy (even a team like RCB is struggling with captaincy errors). And for national team, if India finds a better wicket-keeper, wicket-keeper-cum-batsman, captain, then automatically he will retired or be forced to retire.