The euro survives because leaving the euro would be catastrophic for euro member countries, not because of the advantages that monetary unification brings in principle

In principle, monetary unification has advantages, which are due to the disappearance of exchange rate risk between the member countries of a currency area: Allocation of savings to the most efficient projects, wherever they are located in the currency area; Movement towards federalism caused by monetary unification. But capital mobility between euro-zone countries and the degree of federalism in the euro zone are very low. It is not the usual advantages of monetary unification that prevent a break-up of the euro, but the fact that leaving the euro would be catastrophic for all countries because their external debts or assets are largely denominated in euros. In the event of an exit from the euro, net creditor countries would probably experience an appreciation of their exchange rate, leading to a capital loss on their foreign assets; net debtor countries would probably experience a depreciation of their exchange rate, leading to an increase in the value of their foreign debt in their own currency.