I don't think anybody is saying there is no good music in the last 20 years. (If they are, they'd be wrong.) But, on the whole, there was way more good music in rock, pop, and R&B in the 60's, 70's, and 80's than in the 90's, 00's, and 10's.
In Rock we had The Beatles, Stones, Who, Cream, Kinks, Pink Floyd, Led Zepelin, Black Sabbath, Queen, David Bowie, Metalica, Rush, Deep Purple, Aerosmith, Cheap Trick, Guns n Roses, The Ramones, Talking Heads, Todd Rundgren, Frank Zappa, etc.
In R&B there was Aretha, Diana, Kool and the Gang, James Brown, Marvin Gaye, THE ENTIRE MOTOWN THING.
There were Singer/Songwriters like Billy Joel, Elton John, Jim Croce, Harry Chapin, Cat Stevens, Carly Simon, James Taylor, Neil Young
I don't even like all of them. But, I understand and appreciate their talent. And, I named only a few of the top of my head. There are is nothing to compare to that amount of musical talent from the 90's onward, although the 90's did have some that started in the 80's, such as Nirvana, RHCP, NIN, Soundgarden, Stone Temple Pilots, and others.
For a sampling of current rock and pop, one only needs to watch some of the festival concerts on Palladia. One boring band or singer after another. And the names get stupider and stupider. Kings of Leon, Queens of the Stone Age, Death Cab for Cutie. The one modern act with a decent name, Coldplay, just plain sucks. Whiny pretentious hipster rock.
So, yeah. Music was better back then.