I guess that music is a super good example of what I'm talking about. I feel like it's something of a golden age for a variety of types of music, too. I'll get to that in a second.
Did Metallica go downhill? Sure, absolutely. But that arguably was because they were running out of ideas and trying something new -- why NOT try what was popular at the time? Arguably their downfall was simply running out of ideas.
In terms of a golden age of music, I know that Audioslave got a lot of flack from various sources, but Chris Cornell is really someone I love the music of. He's had some more recent stuff that is more hip-hoppy, and I've even liked that. His various versions of "Wide Awake" are a direct address to GW Bush about 9/11 and Katrina; he doesn't sing about money and ho's.
Anyway, but that's just radio stuff. The really good stuff you find online. I've really gotten into The Queen Killing Kings (The Warden in particular, also Reinventing Language), The Hush Sound (man it's a deep roster of awesome stuff there, when it comes to their music; Medicine Man is really good, as are many others). The versions on youtube really smoosh the sound and it doesn't sound nearly so good.
Then you get folks like Lindsay Sterling doing cool stuff. And then a bunch of indie folks doing covers of game music, like Epic Game Music on youtube, or Chromelodeon (hit or miss, but they've done a few superb tracks), and a bunch of others I can't think to mention offhand.
Okay, but what about the mainstream? How about orchestral. Movie music has never been more varied and interesting. Hans Zimmer just keeps killing it, in my opinion. Alexandre Desplat is a bit too much undertone for me, really, but he's done some amazing pieces such as The King's Speech (whole thing) and the song I'm blanking on from Deathly Hallows Pt 1 where Hermione makes her parents forget she existed. Powerful stuff.
John Ottman has done some fun stuff off and on, and John Powell has shown a shocking range of talents in my opinion. It's a far cry from James Horner, who -- while excellent -- kept doing the same thing over and over again in the 90s. Danny Elfman has done some pretty decent stuff recently, and my absolute favorite standout of the last decade is Ilan Eshkeri. He clearly poured everything he had into the Stardust soundtrack, and it's phenomenal; I'm not sure why he hasn't gotten more work of note since that.
In other words, I think it depends on what you like, and what you're willing to look for. I've forgotten dozens artists that I think are absolutely tops in at least one album or song that are recent. But you won't hear a single one of them on the radio. If you just go by what's on the radio, then sure I can see why you'd think we're in a huge slump.