Then what is holding them back if the iron or whatever has eroded?
LOL man, I have explained in many of my previous posts that Yajud and Majud is actually Mongol tribe who are very famous in raiding established ancient community/nations, which is also feared by Chinese, that is why they also made a wall to prevent them to attack China. Mongol used horses massively in raiding, so wall is very effective to prevent them attacking a nation in full force.
You can see on my previous post that Zulkarnaen has said that the wall someday will be collapsed and other Quran verse stated the time where the wall collapse and Yajod and Majood will pass the barrier and the people will see their arrival as Azab (punishment) from God.
As we know Mongol make many destruction to the Middle East, and particularly the Arab where Quran itself is first delivered to Arab people.
Allah says in the Holy Quran Chapter 21 Surah Anbiyaa verses 96-97:
96 Until the Gog and Magog (people) are let through (their barrier) and they swiftly swarm from every hill.
97 Then will the True Promise draw nigh (of fulfillment): then behold! the eyes of the Unbelievers will fixedly stare in horror: "Ah! woe to us! we were indeed heedless of this; nay we truly did wrong!"
As I previously stated that possibility of Alexandre the Great is Zulkarnaen is strong as he went to Jerusalem and defeat Jews without force and has been said by ancient Jew historian that he believe God of Jew (Allah) that bring his victory. So he may convert to Monoteism when interacted with Jew although the possibility for him to already follow Monoteism when he was still in Greek is also exist as some ancient Greek also worship one God as well as new founding reveal on Greek ancient temple.
I said the wall is between Mountain gap in Himalayan because Alexandre have come to Afghan/Pakistan as well and in the other side of Himalayan is the world of Mongol/Chinese. It is the possible location where local people ask him to make a wall on the Mountain gap that is usually used by Yajud and Majud people to pass through the mountain and raid them. As I said ancient cavalery with horse will be difficult to pass mountain if there is no easy pass