Metallica has always done what they wanted, whether or not it was popular or not. You can think whatever you want, but the general definition by metalheads of "selling out" is doing something other than what you think they should be doing. The reality is, they can do whatever they want to. Just because they changed their sound doesn't make them sellouts.
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While I'm only half serious, here's two things to consider. Reload and Lou Reed.
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A result of musical experimentation. Load/ReLoad came to be because of Lars and Kirk wanting to experiment, and James' love of some Southern rock bands. Lou Reed was because on the night that Metallica played at the Rock and Roll Hall of Fame 25th Anniversary concert, they talked and played around with the idea of recording a record together. Keep in mind, Metallica only provided background music and not much else. It was Lou Reed's project.