Question to speed - the metal era has been going for, what, less than three decades. This is a rather short time and I wouldn't have expected it to have develop much, if at all. To my knowledge the classical music era only got going after it's first 30 or 40 years, its origins were just a slight development of baroque, and the "masters" (Mozart, Beethoven) came into play after 50 years. Don't you think its hasty to judge the development of metal so soon? I, like Final Product, think metal has evolved reasonably well and while it may appear stagnant, I attribute that mainly to the sheer volume of irrelevant bands in the scene. Of course, we could expect a quicker development in than the 18th century (population, information, distribution) but true inspiration takes time. Instead of looking at the crap in metal, look at the leaders over the 30 year history and go from there. One leading band or concept or idea per decade isn't a bad achievement in my mind
As for culture, I agree with your point that the market tends towards homogeneity by nature, but don't the audience have an interest in receiving stuff that is different? If you concede that some bands that know they will not profit from making different music but do so nonetheless, and that people like yourself are interested in consuming music that is different, why should the corporate aspect matter? This sort of sub-culture is always the stage for art's development, and it has been so for centuries - Dushan S pointed this out with the classical artists example.
I think some people are ready for a new cycle, a new phase in the development of music. But it will be a while before most of the public are ready for it - i think this is necessary for such a big change. Hence the change may not come for a while- lets hope it does within our lifetimes.