Let’s not get carried away. By “fiat money” you (and others who use the term) mean ‘legal tender’. Legal tender is whatever the central government decrees will be accepted in that nation for the payment of taxes and must be accepted by banks in that nation for repayment of loans. It is a public currency.

As with any privately formulated currency, Libra would have to be purchased using legal tender, the public currency of some nation. Libra could conceivably become a powerful force in global finance, but neither Libra nor any other private currency could ever make itself into legal tender. In that sense, no private currency could ever replace any public currency.