I would be proud. Abandoning metal because it is "mainstream" is silly and utterly reactionary.
Think about this: historically, where has metal always been most popular, more accepted, and more "mainstream?" Europe, Scandinavia, Norway. I don't think it's a coincidence that this is where the best metal (I would argue) has come from. I am concerned with listening to good, high-quality metal, not with being part of an "underground." The ultimate goal should simply be: more, new, good metal being released. I contend that quality extreme or heavy metal entering the popular/mainstream conciousness does NOT NECESSARILY hurt the potential for more, new, good metal being released, as is seen in the history of metal in Europe, Scandinavia, Norway. I'm not saying it will help metal, either, but if we see a complete absence of good metal being released or an even more precipitous drop in quality, in the coming years, I will eat my words.