I'd say that everyday people's reasoning about ethical matters is subtle, and recognizes the distinction between what is and what should be, and that the hungry person will agree that they eat the bread because they desire the bread rather than because the bread is there.

Actually, from the Epicurean point of view, it could be argued that the herd's ethical reasoning is too subtle -- what plagues people are these ideas which cause anxiety, and so the ideas must be removed so that the person can attend to the simple and obvious pleasures of life rather than fretting over what they have no control over.

I really can't figure out what you're getting at.

I don't quite grant your premise, anyway. The differences across cultures and times make it quite obvious that 'the Good' in the terms you're using is just a group agreement to some moral boundaries. This is not particularly predictable as between groups, or across time. Your syllogism (as such) simply isn't giving what you want it to.

What people deem to be good is predictable.
What is predictable is not arbitrary.
Therefore what people deem to be good is not arbitrary. — Leontiskos

Which people? Predictable by whom, to what degree, and under what circumstances? Is this simpyl a statistical reading of past attitudes? None of this helps... "what people deem to be good" is insufficiently specific, anyway. This is a hodge-podge at best, giving nothing reasonably helpful.

Those boundaries are arbitrary. The collective agreement to them doesn't touch that. To be non-arbitrary you need to be pointing to something which has informed them, which is universally recognized. I see nothing of the kind, until we move into religion. But then, non-arbitrariness is baked in there exactly to get around this problem. Both issues seem to support my contention.