As any good history person knows, there are ancient eras of history knows as the Bronze Age and the Iron Age. This division happened in history because iron was so much stronger than bronze that whatever culture had iron easily beat bronze cultures in warfare. Thus Bronze is weaker than Iron, especially in terms of battle, and Iron should be a higher tier in a game that's all about strategy and fighting.
Also, Bronze has meant the bottom of the barrel for a long time so making Iron the new bottom will just mess up all the Bronze related content that has been put out (like videos and guides) and will confuse us all.
Just my two cents