[Thomas]

It used to be a hugely mainstream genre. Eddie Van Halen, Led Zeppalin, Triumph, Rush, Metallica, and so many others were so hugely influencial on music, and it was so much more mainstream. 

Now, it's niche at best. Metallica carries the torch, to some extent. Tool is still around. Rush was around until recently. Greta Van Fleet is true classic rock, and they're doing great. But these days, mainstream music has changed so, so much. It is now much, much harder for a band to appeal to the masses AND stay true to classic rock while they are at it. 

Panic at the Disco is the new mainstream rock. As much as they are a great band, they aren't true classic rock. Not even close. It's rock with a major pop twist. 

What are your thoughts? And of course, in ten years, what bands do you think are going to be carrying the torch forward?

The thing about Classic Rock is it's just that, classic. No one is making "classic rock" anymore because anything that's new is "modern." Those bands you mentioned are all old; they've had their time. Classic rock used to be hugely popular because that's what dominated the charts. Rock music no longer dominates the charts. Tastes change over time, and I'm sure at some point rock music will be back on the charts. Now we've got indie folk, synth pop, and indie pop on the charts, which I consider to be a huge step up from the bubblegum pop that dominated the charts in the 2000s and early 2010s. At least the Black Eyed Peas are gone too.

Radiohead, Modest Mouse, the Shins, etc are some great rock bands that have filled that void.

I honestly don't get your question lol. "What happened to classic rock?" Nothing. It's just that: classic. You say that it used to be hugely popular as a genre but "classic rock" has never been a genre.