I think a band can surely do whatever they like. If they happen to evolve in a direction that I don't like... well then I will just listen less (or not at all!).
I agree with Loke here - Change is part of the world and I think people should whine less if a band makes an album that is 'different'.
As for what is 'sold out': In metal terms I would equate this with a watering down of 'darker' elements, in order to appeal to a mainstream audience. It doesn't automatically make something bad, but it is generally less likely to appeal to me personally, because that twist of 'darkness' or 'attitude' is what makes metal so enjoyable (for me).
As for Metallica: I loved the first four albums, was a bit disappointed with the black album and don't like anything that came after... however I've seen 'em live about 6 or 7 times, from the 'Master Of Puppets' tour through to some time in about 2001, and on each occasion I've gotta say they were quite superb ('86 was best though!
). Have they 'sold out'? Maybe. Probably accidentally (their direction is kind of misguided, and who knows what fuels it all?). But does it matter? No. It's a little disappointing for someone like me who can remember the impact of 'Kill 'em All' & 'Ride The Lightning' - I miss all that! But there are people out there who
do like the newer stuff, and that's fine.
Plus... bands get old. Different life perspectives make for different musical inspirations and direction.
Agree 100% with Nighthawk in that what turns me off a band is performance without feeling. Soul-less music must be destroyed!