I’m not making the argument for total amortized cost. I’m just talking about per energy unit cost, specifically in terms of CO2 or emissions.

If the evaluation is total lifetime cost to you, using the equipment you currently have almost always wins out. Same with purchasing more efficient hvac and such: the cheapest/lowest efficiency hvac is almost always the most cost effective. That tide could turn with sharp increases in NG or electric prices… but it hasn’t historically been true for many years.