I liked Conrad's post below, and he's absolutely right: replacing Metallica with some guys who win a reality TV contest and take on the mantle would at this point make absolutely no difference. Hell, if you wanted to listen to new Metallica tunes that don't suck pick up a Trivium album!
Every band wants to “get back to their roots.” And they mean it, they do. They're being perfectly honest. The problem is it's *impossible*for metal bands to return to their roots. They've moved on to a new place in their lives, and so have their fans. They're not scruffy, angry middle-class kids anymore; they're depressed, well-dressed middle-class parents and office workers, trying to figure out what the fuck it all means.
The power of bands like Metallica is irrevocably linked to adolescence and testosterone. It's nigh on impossible to recapture that youthful dream we had when we first heard “…And Justice For All.” The best we can hope for is some songs that aren't godawful, but I suspect that most people just don't give a shit anymore. If I wanna listen to Metallica I'll fire up “Ride the Lightning” or “Master of Puppets” and, just for a fleeting second, feel 15 again. And that's enough. Because pretty soon the real world is going to intrude, and I'll go back to listening to music that means something to me *right now*, at this point in my life.
Just for perspective, there's still a big need for heavy music, it's just that now it's being filled by Hatebreed and Slipknot. They're are this generation's Slayer and Metallica, respectively. And there's still a genuine underground too, in which I still maintain a not insignificant interest.