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Serious question - why didn't rationality kill off this debate?Because rationality is just about making logical deductions. Not preferences, not what life is better, not goals, and nothing. At best it's about how to achieve one's goals more efficiently/faster etc. Not what goals to have.
Rationality as some all-around solution to the worlds problems is a naive pre-modern concept. Tons of 20th centuries atrocities and bad decisions have been performed by highly intelligent people using very rational means to pursue them.
>We have consensus on basic ethical norms
No, we don't, except at very crude stuff like don't kill and don't punch blind old ladies. And even "don't kill" can get tons of discussion and counter-arguments when it comes e.g. to protecting one's country, property, interests, the law, whatever.
>- isnt it possible to identify which courses of action/inaction (regardless of bent) lead to ethical outcomes - and review the results of past actions under an ethical lens?
Even assuming we agreed on ethical outcomes (which we don't) the world is too complex to determine "which courses of action/inaction lead to those outcomes" except in very crude cases. Plus, all actions/inactions have second order effects to other ethical outcomes where people can disagree, ad infinitum.
The past is not some safe guide either: circumstances can and will be different, so following the same action/inaction might bring results now whereas it didn't in the past (and vice versa).
Plus, even more hoopla is not about "ethical outcomes" but about personal interests.
No matter what the "ethical outcome" is, even if we agree on what it is, people can still, completely rationally, want to screw ethical considerations and e.g. take your property, squeeze value out of you to your detriment, etc, if it means they get more wealth/happiness/whatever out of that.