Like I said, human growth is slowing down, and has already stopped in western countries. Obviously, iron doesn't grow back, but we could get iron from sources other than earth, like asteroids or other planets. Plus, there's recycling, so we could probably reuse most of the iron we have collected. We could probably do the same with most metals and other resources.
Besides, iron isn't as much of a problem as lack of food would be, so even if we began to run out of iron, it wouldn't be the apocalyptic scenario that overpopulation is portrayed as. Not saying it would have no effect, but it's just not as bad as overpopulation would be.