> People do care about income streams for their descendants or charitable organizations etc. So it’s not completely ineffective.

It is once you make it to the other side of the marginal utility of a dollar.

Suppose you write a popular work in your 20s. You make plenty from it, and then people keep buying it. As long as they do, why spend your time creating new works when you can drink whiskey or play video games or live in a cabana on a beach? As long as the copyright doesn't expire, you have all the money you need and thereby value having more time over having more money. When it doesn't expire for 70 years after your death, so do your kids.

Whereas if it would expire after 14 years and you want to provide for your kids (never mind that they should maybe make a contribution of their own) then to do it you would have to keep making new works.