There's nothing to defend though. You're walling in this beautiful culture that, if given half an opportunity, most of the people being gate kept would likely grow to love and introduce their own new, unique twists on the things already here. That's a good thing last I checked. Only creation, no destruction.

The reason I bring up different metal groups and genres is that it's basically the entire argument. There's no distinction besides an arbitrary one. There's no unified, defining force saying "ok this is metal, period." There are some INCREDIBLY broad musical characteristics that "may" be heard in metal. Metal is like lewd - I'll know it when I see it.