Imagine a world 150, 200, 500 years from now--when virtually every work of art, every novel, ever movie, song, and recording ever produced until today--and many years after--is public domain. Now imagine you want to play muzak in your elevators, or nice background music in your dental office, or car repair waiting room, or restaurant. Or imagine you want to publish a book (or website) of great paintings. If you want to do these things today, most of the works you'd be intersted in are still covered by copyright. Sure, there are older recordings on scratchy 78 rpm LPs, and musty tomes from the time of the Civil War or earlier--but modern stuff, in color, stereo, hi-fi, with modern acting and special effects--most is still subject to copyright. So to play muzak in your elevator or pipe in nice background music to the ceiling speakers of your waiting room, you have to pay annoying royalties each month.
But even now we are starting to see, with the advent of Google Books, The Internet Archive, and Gutenberg, and so on, increasingly modern books entering the public domain. Imagine 400 years from now, and every movie, song, painting, novel published from the dawn of time, every movie made in the 20th and 21st and 22nd centuries, plus hundreds of thousands or even millions of songs, photographs, paintings, ... and the last 100 years or so is still locked up. Now let's say you want to put up a website the 10,000 great paintings; or stream a music or movie station playing great songs and films--will you pay out the nose for the rights to publish the recent stuff? Well, maybe, but if you have an almost unending cornucopia of great, free stuff to choose from--methinks this might exert a strong downward pressure on the ability of copyright holders to extort much money from you. (And this is disregarding practical problems they face, such as some kid downloading all the world's media into his petabyte thumb drive in 17 seconds via a totally secure encrypted link.)