Coal is a rare resource, tying it to something as widely available as charcoal would be an issue.
However it's not like you use a lot of oil. One charcoal pile makes enough oil for 12 boards, which is 2-4 items.
At the same time, we currently have no real issue of resource scarcity since this is still a test and we are not seeing the correct amount of players. Once those are in and the nodes start getting exploited, it will become immediately obvious that there simply isn't enough coal to treat all the existing iron into steel or more advanced alloys. So charcoal will be used, and many items will be made in iron/copper/bronze.