Let me just turn the table around. Why should intellectual property copyright not last in perpetuity? A core proposition in Western world is that intellectual property is indistinguishable from other properties such as house you own or piece of art you create or your stamp collection or your startups. When you die, you have right to transfer these properties to your designated heir. Your properties does not become a public properties when you die even when it would have been good for the society. Why intellectual property should have vastly different treatment?

BTW, I am absolutely not advocating that current definition of intellectual properties is correct or even fair. I am just saying that assuming if this definition was ideal, wouldn't above make sense?