One kink in your argument is that Libra as an organization is going to hold the money that is collected by selling Libras in “bank deposits and short term government securities” (quoting the recent white paper). As you point our, holding large amounts of money in cash is counterproductive, so the vast caches of cash you see Libra collecting will become a huge source of demand for national governments’ debts.

It will be as though the masses of the world had pooled their money to buy those securities — while at the same time receiving tokens that could be used to purchase almost anything. That would represent an astonishing increase in the global money supply. That is the most immediate impact that the Libra will have if it is as ‘successful’ as you (and I, actually) suppose it will be.