I don't think that's it at all in this case.
If Gretzky put the same effort into preventing goals and digging for pucks in the defensive zone that he did trying to score goals and digging for the puck in the offensive zone....we would definitely remember that for sure.
I mean it's really not hard to remember Forsberg, Fedorov and Yzerman being down low in their own zone fighting for the puck, breaking up plays.
Yet you would have to strain yourself to even remember Gretzky coming back much past his own blueline.
After all, we are talking about a/the hockey genius here and you can't tell me that he couldn't of put some of that genius into the defensive side of the game more often.
I'm sure he could have "dug in the corners" much more than he historically did. Of course, his team probably wouldn't have won as much, but he'd be viewed as a more complete player 20 years later, which is all that matters...