He was slightly more swayed by the moral argument: his children had done no work for this income, so why should they be preferred over the rest of society in benefiting from his creative output?
A tempting response is "So something like min(20years, authors remaining lifespan)" but that could leave early work (often less commercially successful) with an expectation of a longer copyright duration than later work.
A fixed period risks letting copyright lapse in an authors lifetime, which is certainly offputting to anyone who, for example, dislikes fan-fiction. So we move towards "authors remaining lifespan + rough average lifespan" as a compromise.
Seems reasonable, although I find one way to increase my understanding of any issue is exactly this kind of discussion.
One minor nitpick: "Reducing the duration from "Year of Authors death + 70 years" down to, just "Authors death", for example, clearly couldn't lead to any given author producing fewer books."
You can't know that. Lots of make money specifically to leave to children, and for many others it plays a significant role.
I agree about all the other points - there is a lot of unknown in this field. I'm just not crazy about people saying "let's scrap this at least partially working system, and replace it with something I just made up". I'm especially not crazy if they don't even demonstrate a basic understanding of the issues.