That original "impetus" also changed dramatically due to technology shifts in the very next war after the Jones Act was enacted: naval combat shifted in World War II dramatically towards the favor of aircraft carriers and submarines. A merchant marine fleet doesn't need submarines (they don't have much cargo room, do they?) and maybe there's some crazy way that an aircraft carrier deck could double as a modern shipping container deck, but why go to all that expense?
The last war that merchant marines truly mattered to war was World War I and "iconic" moments like the merchant fleet at Dunkirk.
The last war that merchant marines truly mattered to war was World War I and "iconic" moments like the merchant fleet at Dunkirk.