you can only "sell out" when you identify yourself with a specific scene specializing in subculture, namely metal, punk, etc. to name a few... those scenes have very strict rules and code of conduct.
so in a way, there are some bands that can never sell out no matter what they do ... take smashing pumpkins for example... eventhough Billy Corgan is really afraid of comprimising his intelligence to the public's perception, he is in a band that can NEVER sell out. that band has no scene, although it did spawn a fad called "alternative fad" - but never a scene (its debatable though since there's this thing called the 'indie scene' which i personally do not like and do not look into).
nowadays there are really vague music genres that are offshoots of metal, punk and whatever... they are mallcore and bands of those type will never sell-out. Like Korn... while its possible they may compromise their intellectual ability and artistic mission, they still do not have a real scene where they can be accused of 'selling out' on.
so do not worry ... i bet many of your favorite bands wont sell out ... they already either have, or its just its not possible for them to.
and just to clear things up from my side, here's an example:
1.) Metallica - yep, sold out. but that is because they outgrew thrash metal and of course, having more money isnt that bad really... besides, they have enough past album materials to satisfy those who miss the thrash metal version of them.
2.) Wolfgang - nope, they never sold out... sure they got more melodic and began ditching the high gain pedal and even the singer went on singing for a mainstream play... but they never sold out. they were never underground in the first place.
3.) Sandwich - again, nope. i am not very fond of them and i can tell you what i think of them in private, but they never sold out for the same reason as above.
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what's wrong with selling out?
depends.
if you used a scene to launch your career, then you deserve to be forgotten.
if you outgrew the scene and did a U turn with the music ... well, its not really bad, but i really consider you changing your band name just to give respect to the scene that you once were part of. dont carry the same name just to get some underground credibility youve clearly outgrown.