All this bickering is unwarranted. It all comes down to this:
We have to draw the line somewhere. If we let in TOO many bands to the fold, then it ceases to be the "metal" hell. So we just decided that numetal and metalcore are both hybrids with other genres and the former is just heavy rock, and the latter is just heavy hardcore.
And look for many of those bands you listed, opium dude, on www.metal-archives.com I can guarantee most aren't gonna be there. It's an inclusive metal website, meaning if you're not there, you're either not metal or haven't recorded any demos, EP's, albums, etc...
That's the general source and argument ender when it comes to metal. What MA says is typically regarded as canon when all is done. Of course, a few ambient side projects are on MA, but that's because members of said bands are also in metal bands, and said ambient projects are important to the metal scene.
As for us, we're a bit pickier because of that. We don't accept Chimaira, even though they're on there, simply because there's another club for that: Numetal/metalcore club. Now get yer ass over there.