I agree with you that some bands get better with age, but Metallica isn't one of them. There are plenty of bands that just suck when they get older. There isn't anything wrong with that, and that doesn't make them sellouts, that just makes them not good at what they used to be good at.
We can all sit here and say, Metallica sucks now, they should play like they used too. But you can compare Kill 'em All to Master of Puppets and see a huge change between those albums. Does that mean that they were selling out because their sound changed? No, just because the change between And Justice for all... and St. Anger is a change for the worse (not for the better like previous albums, up to the Black Album I might add), people want to say they're selling out.
Ah well...
Sometimes I hit notes only dogs can hear.