As your link says "...Elvis points out that one eighth of the iron in the asteroid belt is more than a million times greater that the estimated iron ore reserve

As your link says "...Elvis points out that one eighth of the iron in the asteroid belt is more than a million times greater that the estimated iron ore reserves on Earth, which may suffice for centuries." By the time we start running out of materials from the asteroids, I think we will have recycled some of the material that was previously mined.