A person's voice is unique to the person. A vocal coach won't make him sound like Dio no more than it would make him sound like Britney Spears. Its still going to be Ozzy but it will give him back the strength and range that old age has taken from him. Its still going to be Ozzy, nothing can change that except death. You say you "love him the way he is" but wasn't the Ozzy from back in the day (with more power and vigor and a higher range and longer and more diverse setlists) better?
I'll agree that Ozzy is such a fantastic showman that when his voice DOES crack I give it not much heed but the show would be that much better without the cracking. It would aso probably give Ozzy more satisfaction in his performances. It would probably also allow him to do more shows on a tour (more shows closer together) and extend the setlist. How are any of these outcomes bad? Can you think of outcomes that are bad?

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Tony Iommi = God, his arch angels are Ozzy Osbourne, Dave Walker, Ronnie James Dio, Ian Gillan, Dave Donato, Glenn Hughes, Ray Gillen, Tony Martin, and Bill Ward (bassically everyone who sang on a song labeled Black Sabbath)