The whole purpose of sacrifice is that it should give a reward from God or the gods. Good harvest, good luck in war, good health for your herds, etc. The part where the angel is blessing Abraham after the whole ordeal and swears that he will be rewarded, I interpret as a reaffirmation that Abraham will reap the full rewards even though he didn't sacrifice his child. Ancient worshippers didn't only expect a reward for sacrifices, they demanded it and considered it a bargain with their deities that the deities had an obligation to uphold.
So you can imagine a story teller thousands of years ago telling the other people around the bonfire "And even though he didn't make the sacrifice, he still got more than the full reward".
> The reason I think you shouldn't sacrifice humans now is because I value human life, and that I think it would be wrong to deprive that person of life to appease a God that I do not think exists.
And what proof do you have? We know that the tribe two rivers away sacrifices all men they capture from their neighbouring enemy tribe, and they receive great rewards from their god, who blesses them with better hunting (less competition), better luck in war (lesser enemy forces), and more or healthier children (better nourishment). So it is proven that human sacrifice gives you blessings of the gods. If you want to break with custom, maybe our tribe should banish you and get a new leader?
What I want to illustrate above is that it probably took an immense effort for a leader or for anybody, who thousands of years decided to not do human sacrifice, and managed to convince his tribe against all common sense. This in an environment where it had been custom since the beginning of time, and where tribes naturally expanded in population and territory until game started to become scarce and tensions arose. That's why I think the story of Abraham (or whoever it was originally) is so dramatic and has been remembered throughout the ages. It brought a whole new perspective to sacrifice that probably sounded completely insane to the average person at the time. "Why would a god not want his sacrifice? And why would he reward somebody who didn't give him his sacrifice?".
The materialists who read my comment will probably start connecting the retreat of human sacrifice with the shift from hunting gathering to pastoralism, which is a connection I would also like to make.
Edit: I also think it's super interesting that you bring up the authors intent. In my perspective on these old myths, the original intent is almost meaningless, since the stories have been told thousands of times before they were ever written down, meaning the story must have some core that touches the human soul in a universal way. Or probably that this situation happened so many times with different people that there is no "original" story.