The technology has changed. The navies used to be able to protect shipping.
Now the task is much more difficult.
Just as battleships replaced ships of the line, and were in turn replaced by carriers, all due to technology changes.
Maybe there will be drone swarms or some other future magitech being able to protect shipping.
Or maybe the civilization will collapse due to internal (income inequality, widespread employment of AI), external (ecological disasters) or other (demographics, nuclear WW3) pressures before such technologies are developed.