I think there is a definitive line between evolving, and selling out.
Take Lamb of God for example, Wrath was hugely different then anything they have done before. Mixing in thrash, groove, death and core type sounds in a way that really worked. Was definitely an evolution, and a great one at that. It sold well (their best effort yet I believe), but even the hardcore metalheads still loved it. But then take In Flames, nothing new or revolutionized and nothing better in their new stuff. Pretty much it was the same as their old stuff except with more power chords, less solo's, a piss weak growl and almost "emo sounding" clean singing. That is not evolution, that is just making their music worse for the purpose of appealing to the masses (and they still haven't really done that yet).
I mean even the dreaded "nu metal" sell out phase to alternative was at least better in some ways. It got rid of the rap, way more complexity and less boring low power chords over and over again. Who cares if the bands rarely growl anymore, they made their music better. But this was not the case with In Flames and Metallica's Saint Anger (lets take out every solo and do power chords for 8 minutes each song yay!), which are just prime examples of "sell outs".