It also has the issue of calculating negative externalities to add to the "cost" of fossil fuels without attempting to understand the positive externalities.
The reasoning is "we will eventually have to pay for climate change and air pollution, so let's add that to the cost calculation for accuracy." This is reasonable (depending on the accuracy of your estimates) but it has to be accompanied by another calculation -- what would we have to pay for in the absence of fossil fuels that we're not paying for now? Likely quite a bit, at the very least in the loss of tax revenue caused by the economic slowdown. An estimate that looks at the cost of consequences is meaningless if you don't include all the relevant consequences of both action and inaction.
The reasoning is "we will eventually have to pay for climate change and air pollution, so let's add that to the cost calculation for accuracy." This is reasonable (depending on the accuracy of your estimates) but it has to be accompanied by another calculation -- what would we have to pay for in the absence of fossil fuels that we're not paying for now? Likely quite a bit, at the very least in the loss of tax revenue caused by the economic slowdown. An estimate that looks at the cost of consequences is meaningless if you don't include all the relevant consequences of both action and inaction.