Metal seems like the type of genre that will probably never die in the near future and if it ever dies out it will probably be because it evolves into a completely different genre with a similar aesthetic and aim, rather than dying out due to a lack of interest.
That said, metal is a very tradition-oriented genre so people are probably going to be listening to the same old Death, Atheist, Cynic, Darkthrone, Burzum, and Mayhem a few decades from now just because of how revered and important they are.