Welp, there will always be some great bands that you can consider as headliners, but we must always take into consideration that everything is a product of its time. Of course nowadays there are bands that sound like the older ones, but they do not correspond to the needs of the current audience (not all metalheads are over 40 years old). You can reproduce old Iron Maiden's sound, but it won't be as cool as it was in the 80's (for the mainstream audience, I mean). It will be appreciated by a part of it, though. And concerning the newer bands, the trends and the change this music is always going through, you can say "fuck Slipknot" (and I guess you're right), but they just happened to exist at a time the audience needed this kind of sound, like it or not. Metal will never die, but it will always experience change, while leaving behind some real gems defining its specific eras.