The relative quality of music over time stays the same. I'll add a disclaimer to point out that the choices in music I produce here are a result of being British and will not necessarily apply to other countries, but I'm sure the same concept applies.
When someone talks about "music in the 50s and 60s", they generally do not mean Jo Stafford - You Belong to Me, Lita Roza - That Doggie In The Window, Cliff Richard & The Shadows - Bachelor Boys, Engelbert Humperdinck - The Last Waltz, or The Crazy World of Arthur Brown - Fire. You might have opinions on those songs, good or bad, but those are generally not what people mean, despite all being number 1 at some point in the 50s and 60s.
Instead, they will refer to any number of Elvis Presley songs, Del Shannon - Runaway, Danny Williams - Moon River, The Tornadoes - Telstar, The Animals - House of the Rising Sun, The Kinks - You Really Got Me, and any number of Beatles hits. It's unlikely that the same person will refer to all of these, but it's much more likely than anything I mentioned in the first paragraph.
Similarly, in fifty years time people will be looking at the music of 2000s. They won't be talking about Hear'Say - The Way To Your Love, Daniel Bedingfield - Never Gonna Leave Your Side, McFly - I'll Be Okay, Kanye West - Stronger, or La Roux - Bulletproof. Again, you might have opinions on those songs, good or bad, but those will generally not be what people mean, despite all being number 1 at some point in the 2000s.
Instead, they will refer to Ronan Keating - Life is a Rollercoaster, Enrique Iglesias - Hero, Eric Prydz - Call On Me, James Blunt - You're Beautiful, and Snow Patrol - Chasing Cars.
The same is true of any era. 90% of everything is shit. When you look back, you only look at the 10% that isn't shit.
And it doesn't just apply to music. Take games, for example. 80s games were so good, right? I mean, you had After Burner, OutRun, Galaga, Elite, Chaos, Chase HQ, Dizzy, Alex Kidd, Space Harrier, Joust, Spy Hunter, Gauntlet, and of course the big three of Mario, Donkey Kong, and Pac-Man.
Nobody mentions Streaker, Grand Prix Tennis, Terminus, Soul of a Robot, Ninja, Lazer Wheel, Omega One, Sarlmoor, Rockford, Voidrunner; and when they talk about Fido, Action Biker, or Weasel Willy it's to say how utterly shit they are.