The only reason why I see underground metal should try to unite with the mainstream is so that the best artists can become more widely recognized, get bigger recording contracts, and ultimately have more resources to devote to making music. However, the technology used in the music industry, such as music production equipment, should fall a lot in price eventually to the point where everyone can have their own studio on their home computer, so perhaps the best thing to do is just to wait.
Relatedly, I think sub-genres like black metal really took off in the underground because poor production quality really lends itself nicely to certain types of dark music like that, and since most of the metal musicians had very little resources at their disposal they were forced to play those types of dark metal. With a much greater access to technology, underground metal artists could manage to experiment with many other different types of dark metal, as well as different types of lighter metal too, and perhaps some of these would have greater mainstream appeal than ideologically-extreme sub-genres like black metal.
PS - That British show was carried for a time here in Canada and I couldn't stand it, I think it's the worst comedy show I've ever seen!