Compare metal to punk to disco for a minute here. All originate at a time when rock music has stagnated in the late 70s. Each seems to have a short lifespan of a few years, stagnates, then becomes something new. Heavy metal->Thrash, Punk->Hardcore, Disco->House music. Then if this new form will stagnate eventually, and will either have a rebirth or not. Hardcore, no. House music, I don't know anything about. I'm assuming not. Instead we get bastardized versions or the orginal, which explains pop of the '80s and pop-punk. Metal had enough possibilities to go through DM and BM, and maybe that's it. We've already had several bastardized versions of metal, from hair and glam, to nu-metal, and to a somewhat lesser extent norsecore and metalcore.
Basically there's no further extreme for a genre which has a relied on extremity for expression. There's already been a Nattens Madrigal and Transilvanian Hunger. Vocals can't get anymore extreme, guitars can't get any more distorted. Now the only extremities are speed and technicality, in which the limits have basically been reached within the last few years, with bands like Origin, Gorguts, Necrophagist, etc. Look at what Blut Aus Nord did after they reached the limit of what they thought they could do with metal?