A brief note: I’m not 100% sold on the many-worlds hypothesis—Bohmian interpretations strike me as similarly plausible, but I’m not going to discuss this right now because I doubt I’m educated enough to do so at a high level that doesn’t just retread old arguments. With that out of the way, let’s assume many-worlds is correct.
Given the existence of many-worlds, interpreting making a decision as “Choosing your own Everett branch” is not correct for one simple reason: In any case in which your decisions depend on something going on at the quantum level, you will simultaneously make every single decision you possibly could have made. There’s a sense in which you’re accidentally making the error of importing classical intuitions of “One world” into many-worlds—in this case, the mistake is in believing that there is only one you, who can only make one decision. The reality is that all possible worlds already exist: Everything that has happened or will happen is fully captured by the mathematics of quantum mechanics, and you can’t change anything about it. You can’t change what ever has or ever will happen.
Now, the question becomes the same as for any determinist universe: whether or not determinism, and the fact that all decisions you will ever make are fully predictable by mathematics, actually makes ethics pointless. In this case, I suggest looking back at Yudkowsky’s post on dissolving the question of free will, and then posting your answer here when you think you’ve got it. It’s a good exercise, since it took me a while to figure it out myself. I look forward to seeing your answer.
You’re right, although 1850-1900 captures the Second Industrial Revolution.