If, as you suggest, the artisit's rights lasted 'in perpetuity' that would stifle creativity. One reason that modern music is so diverse is that most of it can be traced back to the classics (Bach, Beethoven, Mozart) or early jazz/blues. This enables tunesmiths to bypass the copyright laws on the basis that the music was prior knowledge - so just think of where we would be if those great works were still copyrighted?
Or think of all the literatry devices borrowed from Shakespeare, Dante, Wordsworth. Where would many of the the great novels of last 100 years be (there would obviously be many but nowehere near as many as we have)?
The question is where is the balance? I think the current timeline is set for the reasons that Rob suggests - that the direct family of an artist can benefit from the artist's skills. I liken this to a guy who starts his own business and passes the business to his heirs who can then do with it what they want. With a copyright of 50-70 years this seems OK to me.
Music is a difficult example for comment, because I think it is more a matter of genre than of individual works. Blues and jazz have developed (some would claim) beyond their starting points and travelled to a place where the old bluesmen of the 20s and 30s would feel themselves strangers. The origins of much jazz are basically suspect insofar as individual tunes are concerned; from military music via church, classical, folk and African bases we arrived at an amalgam that simply worked better than anything else that was going on at that time. Did W C Handy really write that many blues? Who knows? Perhaps as many as the white DJs did when early black RīnīR had to credit some of them (for doing zilch) on the labels just to get airtime? Exploitation ainīt nothinīnew.
But thatīs a problem associated with music. And even there, people do sue for blatant infringement of copyright and WIN. Even in literature the same goes on: the da Vinci Code was an example where someone saw an opportunity of using law to try to make money... the trouble with those sorts of fights is that they are clouded in doubt, the very doubt that makes them expensive to fight but victory possibly very rewarding. The same does not hold for photography: it takes little argument to show if a picture is a rip-off; itīs in your face, as it were.
The timeline, as you called it, is the problem here. Why should it have limit?
Neither do I buy that, because St Ansel made a particular shot of a particular lump of rock in specific circumstances, it has affected the rights of any other photographer to use his tripod holes to do the same - they never can shoot the same shot again because the conditions are unique every time. Copying one of his prints or negs, however, is a different business! But respecting forever the rights within the Saints original work cannot hinder others from exercising their own vision should they have one. That a photographer has made a specific photograph of a mountain that always existed is not the same as a photographer making a photograph of something that only comes alive via his intellect in imagining the composition/concept in the first place, in the juxtaposition of objects and/or ideas not otherwise so positioned. But as original work both types are deserving of protection.
But hey, we could throw this around all day and possibly agree on some points and never on others; in short, as long as you are able to use your own creativity - should you have any - anotherīs copyright isnīt going to hold you back from accomplishing something entirely your own. It is only the drudge, the no-hoper who will feel resticted because all his intellect allows him to see is that which another has already done; for, and by himself, he sees nothing.