"I have a theory I fervently hope is wrong: At some point, a society may inevitably become ethically advanced enough that it no longer has what is necessary to survive against the barbarians."

It is wrong.
First, you need to indicate a society which is highly ethically advanced. Is American society more ethically advanced today as it was 100 year ago? I would say a test of an aspect of a highly ethically advanced society is if the police don't exist, do the people still follow laws? Some modern Americans will riot and loot if given an oppportuny- this doesn't help make the case that American society in general is very ethically advanced.

That UN personnel will riot and loot doesn't indicate a highly ethically advanced group of people. That UN personel will buy sex from girls with food doesn't make a case for ethically advanced.

Nothing about the 4th Estate [MSM, etc] indicates to me a very ethically advanced group of people. Even our Judges and of course Politicans don't lend themselves as high examples of ethics.
No, in fact, any example high ethics is almost a joke in our society and criminal behavior is glorified. Perhaps, only perhaps, the business sector of our society has the culture of highest example of ethics, yet as rule it's villified and is worsen by this constant attack.

What about the public sector? Well, I would say that a group of people who were highly ethically advanced, wouldn't need a zillion rules and massive amount of supervision. Need I say more?