With the whole "how much did death affect popularity?" discussion, I think it definitely plays a huge part. Whether it's Kurt Cobain killing himself, Jeff Buckley drowning or Elvis Presley taking an overdose. Nobody can take away what they made musically, but I think people in general get very affected by death, and we're very good at uniting over it. Similar to how a death in the family can unite the rest and make them stronger, I think a musician dying can have a similar affect and unite a lot of people over their appreciation for said artist. I don't think it's as simple as saying "he died, that's the reason they're big" (as in the ONLY reason), but I do think it plays a significant part. Maybe they would have been just as big if they were still around now, maybe they would be even bigger, but we won't know.
A lot of musicians die and their music fade away though, so to become iconic you must have done something that resonated with people.