The 2nd registry can be thought of as along a spectrum of choices.
At one end the U.S. could expand the U.S. flag fleet by providing financial inducements to the Marshall Islands ships to flag but with no other restrictions wrt crew etc. Or with a higher subsidiary the U.S. could demand that the senior officers be U.S. Or keep the current system with the highest costs and with top to bottom U.S. crews.
Fleet size and degree of control are inversely related. The issue is not that people don’t understand that.