For me, it is pretty simple. On one side, you have the Beatles, Stones, Dylan, Creedence Clearwater, James Brown, Steely Dan, Joni Mitchell, Van Morrison, Neil Young, and many others recording at the height of their artistic powers.
Who would like to try composing a list to compete with that in the second category (post 1977)? Yes, punk and alternative exploded around this time, but how many really believe that the Clash, Sex Pistols, and X are far superior artists to their forebearers, the Velvet Underground, the Stooges, and the New York Dolls?
Normally, I have no patience with those who insipidly claim that older is better by definition. Moreover, I believe that there is plenty of good music at any given time for anyone with the determination and resources to find it. But in this case, the evidence is overwhelmingly on the side of the earlier period. The Beatles and Stones alone seal it--add in the others, and it is no contest.
And let's not even get started with jazz...