It's arguable that after the Brenton Woods agreement that the world did in fact have a single global currency. All other currencies were fixed to the dollar which in turn was pegged to gold (remember foreign central banks could redeem dollars for a fixed quantity of gold even if individuals could not).
Pegs have always been fragile and failed, and that is exactly what happened.
That is an argument on why any single system controlled by people (whether a few or many) will almost always fail, not that we didn't have a single global currency for a shortish time period.
What I'm arguing is that separate "pegged" currencies are not really one currency after all (and can't be). Relevance to side chains.