> Once the direct subsidy of nuclear via the military's nuclear weapons buildout went away, new plant construction... went way.
I don't think there was ever a direct subsidy of civilian nuclear power for weapons production. The ways in which military reactors are run to produce fuel for weapons is different than that of power plants. There are dual use technologies involved, shared R&D costs, and so on, but there is no direct subsidy of civilian nuclear power the way you are implying.
> The capital costs of any traditional power plant are massive, and fix high costs of goods sold for the life of the plant.
I'm pretty sure you are in fact missing the point. The costs of building a modern nuclear plant include a huge amount of one-off engineering because we build so few of them. If we swapped the numbers of units built of coal and nuclear power plants, the cost structures would (probably close to) swap as well, which was the original parent of this thread's point.
Look, I'm not here to advocate for nuclear power. I think renewables + batteries are likely to win on economics in the near future, but with some major challenges (building enough batteries for the long tail of intermittent production lapses seems really difficult to me).
But that doesn't argue against the fact that we shot ourselves in the climate change foot by not prioritizing nuclear over coal power for decades, and I think a big part of that is based on a mistake ("nuclear is expensive") is because of this inability to amortize costs over more units.
edit to add: I think your singling out of capital costs is a strawman of the original comment ("Frankly if there was a COTS design pre-certified, then it might be possible to wipe out a huge fraction of the built cost."). It's unclear whether you are including certification and engineering costs in "capital costs", but the intention of the original point was to include those costs.