If your ethics requires for you to be the first tune whistler in the multiverse, not just in this world, it's not a useful ethics.

The usefulness of the ethics (if that's the right standard to apply to an ethical idea) is not relevant to the example.

That is, unless you want to posit (and we should be super, super clear about this) that there is an a priori principle that any ethics capable of being contradicted by a true physical theory is not useful. But I very much doubt you want to say that.

I think modern physics pretty obviously doesn't add up to normality in a number of cases. Long debates about cryonics took place because part of many people's normal understanding of personal identity (an ethical category if there ever was one) involved a conception of material constituants like atoms such that there can be my atoms versus your atoms. This just turned out to be nonsense, as we discovered through investigation of physics. The fact that atoms no more have identities qua particular instances than do numbers overturned some element of normality.

Given cases like that, how does one actually argue for Egan's law? It's not enough to just state it.