This may, in fact, be the way in which a private-law society would organize itself, though I think the historical record is not in favor of this view. Even granting this view that the majority of disputes will be handled by some kind of pre-existing contractual law, those disputes where there is no contractual agreement act as the “exceptions that set the rules”. It is in the realm of disputes without any prior agreements where the toughest legal problems reside. The resolutions to these most difficult legal problems set the “baseline” or “failure mode” of the entire legal system.
That is, the sorts of contractual agreements that can come into being at all will be determined by what happens in disputes where there are no pre-existing legal arrangements. People will only bother to form contractual agreements if they expect that failure to do so will result in some kind of undesirable “common law” situation in the event of a dispute. If the customary law is sufficient (people expect to get a “fair” treatment from the customary law), then there is no benefit to the costly business of drafting up complex contracts that attempt to foresee every eventuality.
In my view, customary law is the root foundation of all other forms of law, including even statutes and governments - statutes matter precisely to the extent that they thwart or overturn customary law.
Clayton -