Intelligence is a bell curve, so if you're at the far right of the curve, everything else seems retarded. There is no reason to think that intelligence in metalheads is distributed differently than in the entire population.
What might be a difference is that nowadays nearly everybody in Western society a) knows more or less how to read and write, b) has access to the internet, whereas this wasn't the case 50 (a) or 10 (b) years ago.
On top of that, society encourages individual expression. The dictum is "everybody has the right to have an opinion", and "opinions" are considered to be just that: not related to the knowledge/understanding of facts. Opinions have become like "taste" - and de gustibus et coloribus non est disputandum, so everybody feels safe to say their part, since people think "opinions" can't be refuted because the majority of people (that bell curve again) think they are merely subjective.
Society encourages this participation/expression in public debates, since public debates have become a market (Facebook, usercomments on various sites, etc.).
Nothing broke, it's just a side effect of educational and technological advancements: idiots (including the idiots in metal) have gotten a larger forum than the town square or the local bar.