The problem with an argument that welfare states 'nearly invariably' devolve into empire is that even the most cursory examination of the world's welfare states makes it clear they nearly invariably don't.

The reverse argument actually works much better: throughout millenia of metallic standards countries regularly fought wars to plunder gold reserves from weaker states to repay their debts or boost their economy. And didn't even make enough from doing so to be able to afford welfare. Now they don't have to acquire shiny metal every time things are bad, they very seldom fight wars and even relinquished control over the empires they'd built under the gold standard. If there is a causal relationship between monetary systems and empire building, it's quite clear that it works the opposite way round and fiat money reduces the tendency to use military power in service of economic growth.