> They (a) want the income for their legacy in case their products are still in use or appreciated decades later and (b) they want to control the output of their intellect.
Copyright is a practical compromise between society and them; their interests are not absolute.
Isn't the question whether it's reasonable for people to be rentiers? Clearly lots of the population are, but wouldn't it be better if they carried on creating rather than sitting back and doing nothing for the remainder of their place on earth?
I speak only regarding the view expressed in the U.S. Constitution[0]. Other cultures may view it differently, but in my opinion, the US is where copyright is most out of control (save for a few other nations, such as Japan).
> The question of interests is a cultural debate
Not at all, that question has quite real and far reaching economic and political consequences, it's not about endless debating, it's about proper and timely deciding, precisely in the framework of economics and politics within the Constitution.
> their interests are not absolute
The question of interests is a cultural debate, and also not an absolute either direction. In one culture the interests of the author could be held as an absolute; in another culture the exact opposite could be held as the value: no copyrights at all.
That's up to the society to debate. We see considerable cultural variance across the globe on the matter.