I guess I misinterpreted your argument on that point. I'm somewhat inclined to the belief I attributed to you, so I'll try to defend it...
We've been circling the same set of ethical ideas for quite a while now, if not for the whole of our species history. So either we're stuck in a local maxima of ethical reasoning, or there's a disconnect between our capacity to reason about ethics (which we've found possibly the absolute maximum of) and our capacity to implement it (having reached varying local maxima across the globe). I'm inclined to believe the latter, because the problem of ethics is ever-pressing, and yet we've seen no new formulations since (roughly) the enlightenment.
If that's the case, the real challenge is getting a critical mass of humanity up to the absolute peak of ethics, where it's easier to pull up those lagging behind, and where we can more accurately quibble about where the precise peak is.
I don't believe that's happening, for all the reasons it's typically hard to escape local maxima. We've got to take "big" steps to get across valleys, which is not something particularly easy for societies to do, and we seem hard pressed to find a Moses-esque leader up to that challenge.
In this sense, I could understand an argument that said we're not making ethical progress, both because we've already discovered the approximate peak and also because it's seemingly impossible for us to get there.
What do you think? And how did you actually mean progress is impossible?
I really enjoyed this, though I'm not sure I agree with your opening premise. It seems like an individual can subjectively improve their ethics over time, and through empiricism, so can a society.
That said, I suspect an unspoken aspect to your argument is that we have already randomly walked around, if not precisely on, the optimal human ethic. In this way, there can be no ethical progress. It's just a matter of getting a critical mass of humanity to zero in on that location.
I replaced everything except the ceiling lights with no-blue lights and use F.lux on my devices. Now I get sleepy when I'm supposed to and never stay up late.