@DipodomysDeserti Yeah, I'd say the number of kids rocking Nirvana shirts today is 10x what I saw from Pink Floyd shirts in the 90s, despite the same relative time passage and band popularity.
Punk is a good genre for comparison because it's a lifestyle, and many bands are still adding to the canon (even though the heyday of late 70s/early 80s is over).
But Classic Rock, IMO, represents a very specific phase in the evolution of rock. Newer than Bill Haley and Chuck Berry, and past the "four guys singing in harmony" stage of early rock, including early Beatles. It's guitar-forward, blues-inspired, lots of solos, relatively technically complex stuff (not just the generic 12-bar or 3-chord "oldies rock" formula). Often including social protest themes around Vietnam, or just the good old generic "drugs sex and rocknroll" approach.
That's my story and I'm sticking to it -- and I think most radio stations/XM also stick to that idea. It's not to say there aren't more bands today doing that basic same style, I just don't see them adding to the genre as easily as with Punk, or Hip-hop, or other genres that aren't defined by a specific era. I feel like those (and EDM, etc) are open-ended more than a lot of pre-2000 rock, which had a distinct beginning and end. IMHO, of course.