First of all, Danny will rightly be remembered as a legend for the club in my opinion. You don't play the amount of games he has for the club and not get classed in the legend category. Throw in the fact that he was part of that team that finally won at Wembley and that monumental tackle plus his Man of Steel award... if he's not getting legendary status then I don't know who is.
There's some good points made here about whether anyone at the club has the guts to tell him he's done, does he know he's done, and will it tarnish his achievements. I think that agree with the first one. We've seen coach after coach keep flogging him for 80 minutes for a number of seasons (60 this year) and I think we have all seen that's been detrimental to him for awhile now which obviously raises the question as to why the coaches can't see that or don't want to see that. The same could be said for Danny himself. Sometimes it's hard to see your own faults so maybe he's in a bit of denial and thinks he's still offering as much as he once did (certainly not knocking his effort by the way, that's still clearly there for all to see) and he needs someone higher up to make that call. As for tarnishing his reputation, it is possible that it might do that in the short term but I think in the long term people will just remember the good stuff and won't be remembering that he maybe played on too long.