I guess it varies from band to band, depending on which key members are still around. Sometimes the chemistry of the whole band is what defines them. Sometimes there are a few irreplaceable members. Queen isn't Queen without Brian May AND Freddy Mercury. You can replace everybody in Motorhead except Lemmy. You can replace everyone in D.O.A. except Joey Shithead. You can replace everybody in the Dickies except Stan Lee and Leonard Graves Philips. Ramones drummers are almost interchangeable. But, the founding trio of Johnny, Joey, and Dee Dee WERE the Ramones. Although Dee Dee quit performing with them in 1989, he continued to be their main songwriter until they called it quits.
Some groups just lose what made them special after time or some event. KISS is just two old greedy Jews now pretending to be KISS. The Rolling Stones haven't been the Stones since at least the 70's. Metallica just became a glorified cover band after Cliff Burton died, even though Hetfield and Ulrich are the main guys.
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