Voluntarily or involuntarily, he retired after wining his sixth (and final) championship.
Coming around for a seventh and winning it would’ve put him that much closer to Bill Russel. This accomplishment being even greater than Bill’s because it came in a much later era, with a lot more competitive teams.
However, if he’d lost, the enigma that he has now would’ve been tarnished ever so slightly. Right now, never lost in the finals, never even lost the play-offs of a season he played fully, once the titles came in. It carries a somewhat ‘unbeatable’ enigma, which would’ve been lost.
Neither outcome affect his legacy a great deal.