In my mind it's best to go out when your on top. There really is nothing he has touched that his fans and gamers don't enjoy. He can retire as an amazing success and icon.
I disagree. (not that he's going to stop making games now, but with that premise)
Going out on top is nice in theory, but in reality it means that we miss the whole downslope.
Even if he (or any big designer) designer isn't ever going to make a game better then there last one, they can still make games as good as past ones, and better then a lot of stuff lesser talented designers will make.
Me, I'd like to see every television show end only when enough people say it's crap, atheltes quit only after they've been washed up, and artists only stop making work when their audiences are no longer interested. (Assuming plot points or story validity aren't comromised. IE, story being dragged out for more seasons, character meant to die lives because they need to for the plot to stick around.)