Back when my dad was around he brought me up on Motorhead, Ac/Dc, Black Sabbath all the golden oldies in my opinion, I love them and I was strongly influenced by them in my younger years.
I don't listen to metal as much now, purely because I don't get the time but when I do I tend to listen to bands such as Sikth, Parkway Drive, TesseracT, Vildhjarta, Periphery and The Arusha Accord (favourite band back in the day)
It's fair to say that a lot of these bands write the music they do knowing that it isn't going to be hugely popular, but it's the niche that they like to write in, and essentially that's enough to make it pop music.
Take a look at when you're listening to the radio and all the typical stars like Rhianna, One Direction, Rita Ora, and all these other "artists" come on, and then pops up something like Newton Faulker playing percussive acoustic, now if like me you heard him way before he got on the radio you know his stuff was pretty "out-there" it was a niche, but it got popular and now you hear it on the radio (well used to anyway, and I can't think of any better examples) it's been classified as "pop music".
I agree that all music is essentially pop music, just in it's own communities, I remember being back in school we had sections of school grounds for different groups of people, you had the popular kids, nerds, and an area I used to hang around in that we called "The Moshpit" and it was fair to say that Metal, Albeit more common styles of metal (Killswitch, Lamb of God - I used to be quite keen on both for the record), were very much the thing to listen to in that area.
Cai