[Udik]

For example, the OP itself talked about "folk metal", a sub-genre that uses traditional themes, musical styles, and instruments

Folk metal is actually well inside the tradition of heavy metal, that spans from the black/ gothic to the epic and fantasy themes. By the way there's even a more extreme musical genre called "folk": where people from different regions of the world perform actual folk songs strongly connected with their real traditions. Can you imagine, you can actually listen to folk music outside of the rules of heavy metal? That's extreme.

Metal fans love to bicker and debate ... over whether ... still counts as metal at all.

Exactly. A little bit of evolution or variation and people start debating whether you're still in the group or out. That's silly.

And oh yes, I've listened to my share of Yngwie Malmsteen and Cacophony. Heavy metal fans are certainly very proud of the raw technical skill (which is, in fact, just speed) of their players. But it's a sort of pissing contest, who can play the riff or the solo faster is not a good meter of one's musical talent or skill.

[CWuestefeld]

which is, in fact, just speed

Sure, if you're talking about just Malmsteen or Cacophony. But again, you really have missed the evolution. Things have moved well beyond that in the 21st century. For just one example, the band Meshuggah that I mentioned requires the performer to shift fluidly between bizarre odd time signatures. It's debatable how much this improves the musicality, but it's an advanced skill that requires a ton of experience; some small amount of this was visible in the era you're referring to, but the evolution has been, well, extreme.

[Udik]

Listen, I can understand that for a serious chess player, the style of chess has evolved enormously during the 20th century. That doesn't make it less of a strictly codified game. Anyway, it's a matter of tastes..

One last observation. I remember that in the early nineties, a single chord from Nirvana - I was hearing them for the first time- was enough to make me say "this is not heavy metal" (I was actually pretty disgusted by that sound at the time :)). How is it that after more than twenty years (during which I haven't listened to new metal bands), 3 seconds of Meshuggah are enough for my brain to categorize them with absolute certainty as heavy metal?