Today it's life plus 70 years. My guess is they imagine someone creating a hit work then getting killed or dying in an accident, leaving their would-be-rich family destitute instead.
I get leaving the fruits of creative work for your progeny so they can have a buffer after you die, but I strongly disagree that the period should be the length of three human generations. My work should belong to my culture and my world after a decade or so, and really, a decade after publication, not after I die.
It is not natural to treat ideas like regular property. Some degree of ownership and control is needed in a market economy, but even for the life of the creator is excessive and stupid. I can create a cultural phenomenon, but unilaterally decide that it doesn't belong to the culture until 150 years after it was popular? Total bullshit.
Treating ideas as property to be restricted and horded is necessary to some degree in the present world to make it possible to survive from a creative pursuit, but I reject the notion that ideas naturally should be a personal possession, rather than a cultural artifact. Even so, the terms of the system should be tuned to encourage the most and best creativity possible, and allow people to support their families, with some safeguard for death so the family can get back on their feet. Anything more than 10 years is ridiculous. If I die, 10 years living on my creative output is more than enough time for my family to find stability. The idea that a company could just own and horde that work for generations is absolutely disgusting, and completely against the spirit of the system.
Actually, I literally don't understand how this works. How do you determine the "author" of a work created by many thousands of people?