I think the term “sellout” has its roots much further back. It refers to bands making “light” or “melodic” songs that could get released as singles and get radio airplay. Indie Fans can be very territorial, so when a band is accused of “trying to go mainstream” it takes away their street cred.

Looking back, there’s examples like Joey Ramone needed to be held at gunpoint by the producer in the recording studio to put classical violins on their record. He knew it wasn’t true to their sound, but they had to do it anyway.

Also, “cleaning up” a band’s image was seen as selling out. Good example is EMI forcing the Sex Pistols to release “Silly Thing.” They rebelled, of course, and that was the end of their career. (Though it’s not such a bad song).