If I do work today I don't continue getting paid for it 70 years after I'm dead... why should you?
Although I completely agree that the extention of copyright to ever-increasing terms is scandalous and that it should be restricted to the original 10-20 years, I don't buy the argument above. Say I build a house today that I rent out and which generates income for me during my lifetime - should my family be denied that income (or even the house itself!) after I die?
Similarly, if a writer publishes a book today, and then dies a year from now, his family should be able to benefit from his work for a reasonable period of time.
Obviously, the house is a tangible asset while a work of art is not (at least, not in the case of books), but you cannot simply state that my descendants shouldn't receive any income from either asset after I die.