I don't know if I'm repeating what someone else said, so apologies if that is the case.
I don't think modern 'heavier' music will become "renowned classics" for a couple of reasons. Firstly, I don't think it has the mainstream appeal of the classic rock bands you stated, and as other people mentioned, part of the metalhead-culture seems to be not conforming and being underground. If a certain proportion of people do not like a music genre, I find it hard to believe that bands from that genre could be timeless classics. I realise that there are quite a few metal fans on this forum, but as far as I know, the majority of society are now into rnb, pop, indie rock and dnb, at least here in England. If I'm wrong, and more people like heavier music then I thought then good!
However, just like other genres of music, there are and will be legends in the metal genre and all the other subgenres of metal music. Also, I do not think that metal will 'die' away. Very few forms of music do, and are appreciated by people.
So I guess...I think metal music will last, but I doubt it will become timeless classics with non-metal fans. I do think there is a difference with Hendrix as for example, my music teacher appreciates his music whilst hating the vast vast majority of rock bands. I don't see that happening in general with people liking one or two heavy metal bands whilst still listening to a majority of whateverelse.
Personally, I'm not really into metal, but I can see why people enjoy it (I think..hehe) and I don't see it going away. In a good way.