Your summation of the EU actually sounds a lot like what the US started out as, save their constitution appears in multiple treaties. Almost all the properties

Your summation of the EU actually sounds a lot like what the US started out as, save their constitution appears in multiple treaties. Almost all the properties that we think of a constitution as having are present in the EU treaties. The founders certainly never envisioned a single unified fiat currency, in fact they were pretty much of the opposite view: Section. 10.No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility. The founders mostly envisioned that the unifying currency would be gold and silver, it's understandable that the court would let early USD stand as it was effectively gold, however post 1933 (gold becomes illegal) / 1973 (US withdraws from Bretton Woods) it's difficult to make the argument that USD complies with the constitution, or any state law recognizing USD as payment of debt. The constitution would only allow enforcement of the USD at the interstate commerce level but at this point interstate commerce means pretty much anything the Federal gov't wants to regulate that wasn't explicitly granted in the constitution.