A couple of thoughts:
1. His nearly exclusive focus on heavy and speed metal obscures the overall history the genre.
2. It seems to me that there's an explanation for this sort of thing built into the nature and structure of modern culture and cultual mediation. We have now had nearly 100 years of modern popular music, mediated primarily not through rare live shows in concert halls, but through radio and recording (and, more recently, television and the internet). The move from music as a concert driven experience to music delivered electronically has had profound structural effects on that nature of music, which in turn fudamentally alter what music gets made.
The most obvious effect has been the commodification of music. The ease with which music can be packaged, marketed and sold removed many of the entry barriers that in centuries past seperated the wheat from the chaff and prevented dilettantes, dabblers and dead weight from accessing music as a career option. The economics of the old system of mediation to cultural elites through a very limited number of prestigious live venues. When people are able to experience music as art only a few times a year (or maybe a few times a month, at most), there simply is no room for mediocrity (as there is, for instance, on a cd shelf). This ensured a consistent level of artistic excellence and provided an incentive for continued dedication and the maturation of talent that simply no longer exists in the electronic age.
Commodification replaced the artistic excellence with a new imperative: making as much product available to as many people at the highest possible profit margin. The consequences for quality are manifest, but there are other, less obvious results as well. Commodification and saturation placed new demands on the industry. Electronic mediation placed a premium on novelty and marketing; music was absorbed into the "Next Big Thing" cycle of the advertising world, and while there were far, far more windows of opportunity which artists might enter through, each window was also far, far more narrow than had been the case in previous eras. As a result, the other lasting legacy of the commodification of music has been the near complete absorption of music as a cultural expression by the wider culture of youth.
This music-as-youth-culture paradigm shift has implications for the failure of metal to escape the pop culture dungeon it formed in protest against. The reaction of youth (in any generation) to the world it inherits is predictable and well-understood: disillusionment, alienation, anger, revolt. All of these are conducive to the stirrings of artistic creativity. But their very nature sows the seeds of their (creative) demise. Anger is powerful, but untempered by any positive ideals and vision, it is profoundly brittle and completely unsustainable.
Speed metal is a classic study in what happens when the fire of anger goes out. Many artists simply disappear, others, disillusioned by the process, become what they hated and "sell out." Most of the rest take the path of the silent "sell out" and cynically pander themselves to the anger of their audience, descending into self-parody as they hang on to the ashes of their fame by turning their music into some sort of auto-catharsis for emotions they themselves no longer feel.
Even the truly transcendent artists, those with vision to accompany desire and ideals to go with anger, end up being consumed by the system. No matter how much they hate it, if they wish to expose their art to the world, they have to play at least loosely within the rules of the game. There is no time for patient development, for the slow mining of creativity. If you've got the lightning, you have to bottle it now or no one will ever know it existed. Beethoven's 9th was the culmination of a lifetime of artistic and personal growth. Hvis lyset tar oss was the culmination of 3 years of fevered work by a kid barely old enough to be out of college. Beethoven went triumphantly into the night, escorted by 10,000 admirers. Varg Vikernes went into prison exile after stabbing a former friend to death. Perhaps there is no better symbol of what we really ought to do with metal.