For the World to forget the music of the 60s and 70s something better has to replace it. That hasn’t happened. Incorrect. Again, a good proportion of every generation thinks that their music is the best. There are plenty of people my age who prefer Nirvana, or U2, or Green Day to the Beatles -I'm not one of them, but I would call myself outnumbered.
Say what you will about The Beatles, but the fact remains they crammed an entire (diverse) musical career, producing a kazillion hit records and albums, into a time-span that most of today's "megastars" require to produce two or three forgettable hits.
They were and are, with the possible exception of Elvis, a once-in-a-century musical phenomenon.