Ignore what I wrote. I mentioned earlier to @Gorrister that I recalled seeing prominent parent groups coming on television and making a crusade against "corrupting the minds of the youth", which were largely attributed to these three artists. Most of those self-righteous campaigns ended after they died. Another reason might be, if they live on, their work won't be as appreciated as it is now.
Elvis did just fine and he got just as much crap as any of these guys.
As for there career's Tupac and Biggie would be legends regardless though they'd have there up's and down's like any other artists. I mean look at Paul McCartney, he's still regarded as a icon, rightly and he's made some stinkers. Cobain is hard to say, his career was really just starting and he had a very self destructive attitude so minimum you'd need him to kick the drugs and not have a negative view on fame. He did for sure gain from his death at such a young age as other grunge bands were doing better before the suicide. And as others have mentioned this may butterfly the Foo Fighters.