I think if Michael had, god forbid, left us prematurely around the release of automatic, he would have been elevated to icon levels way beyond Kurt. Their body of work was impeccable at that point and Automatic being a final album would still be breaking hearts with it’s poignancy. I think they’d be elevated further by a myth of what could’ve come next and how music was “robbed” of the greatest albums never made.
I don’t think that’s the same with Nirvana. They had a no more than decent debut, one defining masterpiece and a great final album that was essentially designed to piss fairweather fans off. Also, anyone who knows even a small amount about Nirvana know they were imploding by the end and likely would’ve not lasted much longer than the mid 90s at best. Unplugged in New York kind of became the poignant mythic album linked with Kurts death more so than Nevermind and with Automatic being a genre spanning original album about death, think it would’ve been way more powerful.
Kurt will always be an icon and appeal to a disillusioned youth with his tragic story truly immortalising everything. I think Michael in your scenario would have been immortalised in a higher echelon, closer to John Lennon.
Thank god that scenario didn’t happen and the above’s all opinion but I’ll stick by it.