Cobain's popularity is directly tied to his suicide, though. I doubt he'd be as popular, or Nirvana placed on a pedestal if he were still around.
I don't know. Nirvana was a huge, huge band. They became the biggest band in the world overnight because they really took music and punched it in the face; no more Michael Jackson, no more hair metal - let's rock.
Who knows what they'd be if they were still around. It's safe to say they had more than one more classic album up their sleeves though. And in essense, one could also say that's being unfair considering all bands lose some popularity as they go on. People start copying your shit, and before you know it the clones are making more money off of your music than you.
They'd still be considered a ground breaking influencial band whether he died or not. The same way Tupac would still be the most influencial rapper, dead or alive; he'd probably be even more influencial if he was alive because he'd be putting all these wack ass rappers in check.
If Lennon was still alive would he be as popular? Probably not. But that wouldn't make him any less influencial or great*
*despite not being able to sing