well some of metal is pretty mainstream, you see ozzy on mtv, on jay leno etc, and he his supposed to be the metal icon...
its true however that most experimental metal bands would never get mainstream unless they change their sound, but that happens to all kinds of music, not just metal. every genre has more and less accessible music.
still compared to jazz,classical, progressive music, metal is more mainstream
you got maiden, sabbath, ozzy, metallica, megadeth, pantera,etc all big names, now its true some of those opted to become more commercial, but at one point, they were still metal , shitty metal but metal
plus you still can see a bit of metal on mtv, jazz , classical on mtv? you got to be kidding eheh, simple because there's in no image involved, not appelative to young people.
for the more experimental metal bands and other "sophisticated" genres of music to become mainstream, there would have to a great change in society, industries all want to make money, kids have no interest in good music, they only care about image so they could score with girls, most people want to be entertained by music, media...
music would have to be taught in normal school,i actually had music class in my school , but they should teach with modern examples, you just can't go teach kid to like and play beethoven when he is 12 years old, first go with what is mainstream, make them aprecciate music, then gradually build interest in other more mature styles. i never cared about that music class :/
radios would have to stop playing crap....fuck, its impossible....this would never happen...