For a very long time (70’s through mid 90’s) Rock was king of the popular concert based music, with Heavy Metal being one of the Dark Knights, some may say Joker.
All that shifted as per amount of money made, in the 90’s with the (Breech)birth of Rap.
I think there is still a solid fan base for hard rock among today’s young people, just not on the scale of what it once was. Country Music went through a similar arch.
The genre has reached the mature stage of the lifecycle, like classical music, people just want to hear the classics of the genre, and not the new stuff, that’s why the old bands can still pack ‘em in, even if there are only one or two members left from the original band.