There are two more factors. First, not all electricity generation facilities are the same. There are more efficient ones and less efficient ones and ones, IIRC, kept to kick in when there is peak demand, which is a justification of peak demand pricing. So, a reduction in demand should produce a greater than linear reduction in cost, since the expensive generation facilities don’t have to be used.
Second, if demand is expected to increase, the electric company should be maintaining a reserve to build new plants. At the least, a decrease in demand should delay the need for this, and let the costs of depreciation of existing plants diminish as old, relatively cheap, plants get fully depreciated and new ones don’t come on line.
I certainly don’t know the relative magnitudes of these savings, but they should be factored in.
Electric companies aren’t the only ones to pull this trick. A while ago, during a drought in California, the water district in San Francisco I think called for conservation. There was a big drop in consumption - leading to the same request for increased prices you see here.