There's a simple metric for the state of modern music - where are the modern equivalents, in musical stature, of David Bowie, Pink Floyd, Roxy Music, Bob Dylan, Elton John, U2, Fleetwood Mac, Steely Dan & The Eagles? Answer - there isn't anything anywhere near the pantheon of talent that graced the late 60s to the mid-80s. It's just a phenomenon of history. It's not relativism due to ageing. It's a fact in the same sense that Shakespeare's work outlived that of his contemporaries.