At the negative impact i can say that this will not happen because the needed Darksteel to craft epic/legendary equipments will be way more they will not even think of selling those. Well even if they sold it you will still benefit because if you need it too you can just buy from the market. But i agree on adding other features to the game that will give some dark steel or just increase the dark steel that can be mined like 800-1k per mine. Or just make it a bit higher than the mining rate on MIR4 Global.