Long story short: you can't issue a reserve currency, export that currency in size and peg it to gold. As the U.S. shifted (by necessity, as noted above) from an exporter to an importer, a percentage of those holding dollars overseas chose to trade their dollars for gold. That cycle of exporting dollars/importing goods to provide capital to the world would lead to all the U.S. gold being transferred overseas, so the dollar was unpegged from gold in 1971.
Since then, the U.S. has attempted to square the circle: continue to issue the reserve currency, i.e. export dollars to the world by running trade deficits, but also compete in the global market for goods and services, which requires weakening the dollar to be competitive.
In a global marketplace for goods and services, all sorts of things become tradable, including labor. The misty-eyed folks who are nostalgic for the 1950s/60s want a contradictory set of goodies: they want a gold-backed currency that is still the reserve currency, and they want trade surpluses, i.e. they want to export goods and import others' currencies. They want full employment, protectionist walls that enable high wages in the U.S. and they want to be free to export U.S. goods and services abroad with no restrictions.
All those goodies are contradictory. You can't have high wages protected by steep tariffs and also have the privilege of exporting your surplus goods to other markets. That's only possible in an Imperial colonialist model where the Imperial center can coerce its colonial periphery into buying its exports in trade for the colonies' raw commodities.
And very importantly, oil is no longer cheap. The primary fuel for industrial and consumerist economies is no longer cheap. That reality sets all sorts of constraints on growth that central states and banks have tried to get around by blowing credit bubbles. That works for a while and then ends very badly.
The 1950s/60s were not "normal"--they were a one-off, extraordinary anomaly. Pining for an impossible set of contradictory conditions is not helpful. We have to deal with the "real normal," which is a global economy in which no one can square the circle for long.