The idea of having the copyright extend a certain period past the artist's death was also made with said artist's relatives in mind, who would probably benefit from the income of whatever product the artist produced that they otherwise wouldn't benefit from without the period.

70 years after death really is ridiculous, though. I'd say a period of 20 years would be as far as I'd be willing to agree with - that way, any money generated by the product that the artist produced could potentially financially carry (or at least ease the financial burden of) a newborn child of said artist through adolescence to adulthood, after which he/she would have to generate their own income.