When it comes to bands like Metallica, green day or nirvana it's hard to say they sold out. Their music for the most part stayed the same but became more mainstream. This is beautiful for them but they don't like being labeled as sellouts since their music has gone mainstream and those fans that loved them before they were winning Grammies and being more recongnized tend to look at them as selling out since their fan bases have quadrupled in an albums time
Everyone one of these bands had huge departures to indicate a switch from music that was based on creating music as its primary goal (see Glen Gould) to its primary goal of making money.
Metallica- the black album had more radio friendly songs than 6-9 minute anthologies. The 3 minute pop song was a big switch.
nirvana nevermind - the band went from indie records to studio albums that sounded canned and polished.
Green day Dokie was probably the start, but seriously, no gives a **** about them. Bunch of turds.
I don't care if they sold outs. If you make money doing what you love, so be it. But there is a philosophical point that describes the purpose of making music in multiple ways. Pianist Glen Gould talked extensively about this subject and quit performing live because of a conceived violation of principle. He maintained that there was a primary motivation for creating art and music, which was/is the creative side, which is the soul of art, taking nothing from the void and creating something. The interior.
The secondary motivation was from outside influence, the exterior. This is basically any influence like money, others etc. Gould quit performing because he claimed in his music, the idea of paying bills, selling out halls, creeped into his music and thus corrupted the primary motivation.
This is the essence of selling out.
So when Metallica starts making radio hits after making tons with "one," their motivation change is obvious.