[EtellaObtaite]

why is the argument"Ethics must have...." "demonstrably weak?"

In the case of the Dhamma (meaning without making this point to a Western philosopher) I would say that since metaphysics are rejected, ethics which rely on a metaphysical assertion of any kind must also be rejected. Therefore, any ethics which we do not immediately discard must be based elsewhere than in a realm of ontology, et al.

(Such an ethics is discerned using epistemology, instead of metaphysics. It's a sort of Moral Naturalism, although to be precise I would argue for a Moral Particularism - and I would require the mind be considered a sense; Western epistemology often treats the mind as separate from the five sense, but the Dhamma does not make such a move. Altogether with these preliminaries, Buddhist Ethics can be constructed.)

It is a field rife with debate, so of course I'm not "right"; metaethics is a good introduction.

The philosophical gyrations might seem excessive, but replacing "god did it" or "kamma-as-moral-gravity" with a robust moral construction takes some careful work.