There is no "super clean natural gas plant" you're still burning a nonrenewable hydrocarbon and letting its carbon go into the atmosphere

I think you missed the point of the example.

The question I'm asking (and that I think the study was trying to ask) is "If we have limited resources to apply, how should we try to minimize CO2 emissions?"

The point I was trying to make (and that I think the study was trying to make) is that replacing the plants that emit the most tons of CO2 per MWH is a decent place to start, especially when you know you don't have the money and political will to replace them all.

(But you probably ought to put other factors into the math, like that you need X MW of solar to replace 1 MW of fossil fuels, to account for the fact the sun doesn't shine at night -- and the exact value of X differs due to local conditions like latitude and weather.)

Saying "I wish we had the money and political will to replace all fossil fuel plants" or "We have a moral imperative to replace all fossil fuel plants" is...not productive. Your wishes or moral views don't change the fact that we don't actually have that much money and political will.

There will be some bureaucrat or politician or somebody somewhere who at some point says "Okay, we know how much money and political will we have for this CO2 emissions reduction thing. Where should we spend it? Let's see if anyone's made a study."

And when that decision maker goes off searching the literature, we really want them to find a study that gives an honest, non-misleading analysis -- with the example numbers I gave, that would mean closing the coal plant in Indiana is the optimal play, even though that state's climate and weather is less ideal than south or west.

If the study uses misleading math and the resources get applied in a sub-optimal place -- if we could have gotten rid of X tons of emissions with the money and political will available, but because of the bad math we applied it in the wrong place and got rid of Y tons instead, for some Y < X -- that just seems like unnecessary societal inefficiency and stupidity.