The reason that I find your argument invalid is that there is no ability to sink ships for materials immediately. You have to fill those "slots" and only then can you sink for materials. And if you wish to get materials, you fill those slots exactly twice. Once with cargo, once with materials and chests.
For this exact reason, it is my firm belief that for each slot you should be able to port and receive one cargo/loot and one material. Besides this, the game states in its mission that the goal was to bring back the game as Disney's POTCO was. Disney had the loot system the way it was, where you port and got cargo and mats. It should stay the same here.