Bands that have lost key members
 

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Uh, where to begin.

Pantera - Dimebag Darell
AC/DC - Bon Scott
Lynyrd Skynyrd - Steve Gaines, Ronnie Van Zant, & Allen Collins.
Tragic story behind Lynyrd Skynyrd.
Ozzy Osbourne - Randy Rhoads
Metallica - Cliff Burton
Led Zeppelin - John Bonham (Who knows how much more they would have gone)
Nirvana - Kurt Cobain
The Who - Keith Moon
The Allman Brothers - Duane Allman
The Doors - Jim Morrison
Kiss - Eric Carr
The Rolling Stone - Brian Jones

Some don't exactly "go flat", they keep going, and move on, but
overall, a band is never the same after one of their own dies.

RHCP - the tragic death of Hillel Slovak
Dinosaur Jr. - J Mascis's ego and Lou Barlow's pussy-ness couldn't coexist
Incubus - Dirk Lance leaving the band :(

Smashing Pumpkins?

Probably more to do with the fact they just ran out of ideas and creativity after 3 or 4 good albums than the member change.

Whilst the change in membership would have scuppered the Melon Collie-era momentum, I do agree that Billy Corgan's hubris did them in rather than any loss of "key members"-considering the only key Pumpkin was Billy Corgan.

Why Alice In Chains would ever even think about getting back together after Layne's death I will never know.
All though most people never looked at Blink 182 to fondly it seems like they completely embarrassed themselves after they kicked Scott Reynor out.

Misfits are a given.
D. Boon
Weezer was never half as good without Matt Sharp.
Whenever Hiro Yamamoto left soundgarden and Ben Shephardjoine dit was a turn for the better.

Agreed, which is weird because I like Travis Barker and I believe him to be more talented than Raynor...

Jimi Hendrix Experience anyone?

No mention of Black Sabbath, eh? I'm just not a Dio guy. The only Sabbath I dig is with Ozzy on vocals.
Same story with The Misfits, AC/DC, and Lynyrd Skynyrd as far as vocalists go.

But when Faith No More picked Mike Patton, it was infinitely for the best.

Whilst not the front man lee Roth was I'm sure we all agree Hagar was a far superiour vocalist,

Acca Dacca were not a weaker band without Bon, just different. They actully missed Phil Rudd more when he had his hiatus.

Slayer without Lombardo were not as threatening a line up.