I agree, phases make sense. I myself like to think of the situation you're describing as the coal costing the utility $15/ton or whatever while the rest of us

I agree, phases make sense. I myself like to think of the situation you're describing as the coal costing the utility $15/ton or whatever while the rest of us pay that $100-120/ton to repair the commons. But certainly different sources will become less viable at different extra costs, and perhaps for coal $15/ton might as well be $100-120/ton.