Personally, I think what I would consider "good music" and the mainstream have never been further apart.
But that's entirely different from music of today vs music of the past.
18-40 year olds are too tech savvy to be constrained by the mainstream radio scene anymore and thus that scene is forced to cater to 10-18 crowd and 40-55 (or so) crowd and unless it's pop music, it's almost always gong to be what I consider "bad" (just cuz I consider it bad doesn't make it objectively so
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I think there's just as much good to great music, if not more, out there today than at any time period. It's just that it takes work to find it and it's probably not going to be super popular where as in the past it was much more likely to be popular.
The music scene is so different because of the computer. Nirvana isn't possible, anymore. I don't mean the style of music but if there's one band that defined the 90s or first half of the 90s, everyone would say "nirvana." They changed the entire scene (we're talking impact, not quality) and were really the voice of a generation to an extent. That can't happen in today's world. Sure, Nirvana might have broke out and become a success but they never would have had the impact in today's scene as they did in the 90s. The same is true for Miles Davis in Jazz, or The Beatles, or Pink Floyd, or Madonna, or NWA, or anyone else.
We shouldn't be comparing it to the past in these terms because the game has changed so drastically.
edit: There's also the issue that bands today have taken so much from those before. Sure, the guys from the past took from those before them too, but the change was much more drastic than today. Bands today can't escape the fact that a lot has already been done. It's hard for a artists to sound as fresh as Pink Floyd or Madonna, or Radiohead, or whomever else. Hell, Mile Davis invented styles of jazz; it's kind of hard to invent a new style of music now.