Rock music lost it's mainstream appeal by design starting in the mid-aughts. Rock radio as a format hasn't seem to support or push new bands in probably the past 15 years, deciding to switch over to a classic rock format. I am of course generalizing to a point but that is just my observation as I went through my 20s for much of the noticeable decline of rock music at the mainstream level. File sharing, record companies not investing in bands over time, the consolidation of radio with the Telecommunications Act of 1996 all certainly played a role as the rock stations all became more and more the same over time. I do also think that the record labels also decided to make a move away from rock music as it has been a genre aimed at and dominated by White artists so I do maintain it's decline is part of the Great replacement.
That's not to say Rock is dead as a genre, it's certainly disjointed and broken up to many different genres and sub-genres as my generation and the subsequent generations seem to have a proclivity for. Rather than let everyone enjoy a band the fractured mentality is for that band to stay within the narrow confines of the sub-genre of the gatekeepers. There are some good channels on youtube that post desert/sludge/doom rock and bandcamp is also a solid resource as it allows for bands to self-release albums. Of course the exposure is much less than it would be on a major label but good music is out there.
As far as Pantera - they have reunited minus the departed Abbot brothers and the tour seems to be doing well. They just played Madison Square Garden and I believe there were 18,000 tickets sold. Phil has stated he is seeing a lot of younger fans at the shows. There is still a target audience for rock music and hopefully Pantera's reunion will inspire younger kids to get into bands to make good, unapologetic, aggressive rock music.