At a time with far less globalization. The world will live, sure, but it would definitely be a major change if the movement of goods around the world also necessitated arbitrage due to currency fluctuations in whatever local currency was being used to negotiate its passage at each stage.
Has it? It’s pretty much been a gold-backed currency making gold the defacto reserve currency. Curious whether the USD has been the only fiat reserve currency.
Global currency reserves are currently ~60% USD, ~30% EUR and 9% GBP/JPY/CAD, USD use as a reserve currency has been trending down for decades now. As for previous reserves I would guess before the Euro the German Deutschmark and French Franc would both have been used for reserves instead.