I mean, to be a musician of that level is to be part of a business. That's the purpose of a business: to grab cash. I get this as a definition, but it's so subjective as far as what "at the expense of the art" really means, that at the end of the day I still think the term is mostly a way of saying "I don't like this anymore". A lot of the bands mentioned in this thread are ones where, if anything, it looks like it really happened the other way around -> they made something commercially successful then used the freedom granted by that success to create whatever they wanted. Opeth is a great example of that. To me, to "sell out", by the definition given here, would be for them to just keep re-making the same albums they already made.