[danans]

That's true if the minerals extracted are consumed completely.

Metals however, are often the most recyclable materials we use, because unlike carbon-based materials like plastics, metals often have useful properties in their elemental state, or as alloys that can be melted down and reused without the loss of those properties or of much material.

Most aluminum, for example is recycled, because the cost of recycling it is lower than the cost of mining new material.

[throwaway4aday]

The concern isn't that we'll completely deplete the ocean, that would be a monumental task. The worry is about what happens at the "mining" sites. Those are going to be in continuous operation for a long time.

[danans]

Assuming that the magnesium is coming from the seawater and not the seabed, would we not expect currents to re-balance the concentration? In fact, we would design the facilities to ensure that it happened.