Speaking from an entirely generic viewpoint… probably somewhere between forgotten and almost-forgotten but still turning out albums.
It’s easy to forget how many white-hot superstars of a moment burn out in the next, no matter how much promise they show or studio backing they have. It’s also easy to grossly romanticize celebrities who die young, badly, or at some early peak.
I think James Dean would have gone the way of Sal Mineo (down to the sad, dirty death, probably at the hands of some rough trade.)
I think Marilyn would have ended up lumped in with the other suicide blonde D-cups.
I think John Lennon would have persisted at the “quirky ex-Beatle, doing weird stuff with that strange wife” stage, no more popular or beloved on a large scale than Harrison.
I think many of the dead rap and hip-hop stars who went out in a blaze of gunfire would be forgotten, eclipsed, minor playahs on the P. Diddy spectrum or just churning out albums no one but a small fan base keeps buying.
Concur with LN on Buddy Holly and Richie Valens and so forth - basically forgotten and lumped in with a dozen other performers of the era. (Put it this way: If Kris Kristofferson had gone down on that plane, as he nearly did, he’d be some huge country/pop avatar instead of just another kountry krooner of the 70s.)
And on a more serious note, I think JFK would have Nixoned out, because it was time for a Nixon, any Nixon, even a young, handsome, Democratic one. (I recall my mother’s opinion on this one, around 1975: “Being a hero is dying at the right time.”)
The only thing I really remember about Aaliyah is that her insistence on bringing all of her mountain of luggage on a small plane may have contributed to the crash.