The problem i have with metalcore is it took off 20 years ago, and hasn't died out since. So its grown stagnant because there was no decline from a mainstream scene, so the metalcore purists are still mixed in with the trendy genre hoppers. It hasn't been purged and purified by almost dying out, like other forms of metal. All forms of metal had a period of decline in popularity.This decline in popularity weeded out the wanna-be's and left the people with a true heart for the music, and its evolution and growth, to dominate the scene. Metalcore is long standing scene genre still dominated by trendy types. There is little growth in the scene as a result.
I think black metal suffers from this some, as well, but since it was never as popular there was not as big of a mass of trendy mainstreamers posing as black metallers, so less need to purge. But it still does have its posers that need to be purged. Too many 15 year olds who got a guitar for Christmas and wearing their Halloween makeup and recording 'albums' on their parents old boom box to record 'albums'.