I still think the question of what would Metallica be today if Cliff didn't die is the biggest mystery in modern music history. Would he have stayed in the band? Would the black album have happened? Would Metallica be as big as they are?
Well, I think the overall scheme of things wouldn't have changed. Death makes someone immortal and flawless so to speak, imagine Elvis dying at 27, he would be an iconic legend more than he already is and nobody could ever picture him fat and struggling to sing.
Back when Metallica were young and hungy kids taking the world by storm, who could have imagined the fearless leader James Hetflied cutting his hair? who would have thought the other mind of the band, Lars Ulrich, could become so enraged at Napster? and who wouldn't have laughed at the idea of the solo master Kirk Hammett being sloppy and with eyeliner?
Nothing tells us that Cliff would have stayed "true to Metallica's heart" or whatever, he could have grown tired of the music like Kevin Moore and leave of his own account for all we know. He was a young boy like everyone else, who now are married family men. Like the others changed, so probably he would have.