At what point did it become wrong to expect metal to have a place in the main stream market?

[Seederman]

If the masses wanted it, it would be mainstream. No dis to metal, but the fact is that it is always going to be a niche market, and because its listeners often specialize a couple of very narrow subgenres and exclude others, every band is going to be a niche band. Television doesn't show metal acts because more viewers don't want to see it than do.

To answer your question, metal stopped being mainstream around 1974, when glam rock, funk, disco, punk, and soft rock were catching on. But it will always exist, as a niche market, and many bands will make a very good living selling their recordings and show tickets.

Most of the best music is niche music, it's just the order of the universe.