Fallacy of composition disproves argument that U.S. can rely on customary international law -- even if true, it wouldn't be in our interest if others followed our example

Putting aside for now the potential consequences of blurring the distinction between broadly ratified convention regimes and customary law for other subject areas of concern (e.g., the 1977 Additional Protocols to the Geneva Conventions and the Rome Statute for the ICC), one might reasonably ask what response such a position might invite from other states that are now parties to the LOSC. Could they too circumvent the LOSC’s ban on reservations and avoid its compulsory dispute settlement provisions by renouncing the LOSC in favor of customary law? Even if empirically sound, the argument that nothing is to be gained by the United States in ratifying the LOSC, because all of the best parts either codified existing customary law when the Convention was opened for signature or later (i.e., between 1982 and 1994, when it entered into force) ripened into customary law, must be tested against the fallacy of composition. If that is true for the U.S., wouldn’t it also be true for the 160+ nations that are already parties to the LOSC? In short, do regimes founded on rules of customary law better serve the national and shared interests than those founded on treaties? The common understanding of the fallacy of composition is that what might be true for the one is not necessarily true for the many. If one person in a crowd stands on tiptoes to see better he might be better off, but if everyone does it no one is better off. The economist John Maynard Keynes referred to the analogous “paradox of thrift,” by which he meant that if one person saves a substantial portion of her earnings she may be better off, but if everyone saved as much it could lead to a recession.