I think the steelman pro-nuclear argument is that if we seriously wanted to replace all fossil fuels with nuclear, we'd have to build so many reactors that they become faster and cheaper to build. They also don't require expensive seasonal storage like solar panels and wind would in most of the world or expensive HVDC lines from always-sunny areas of the world to the areas requiring seasonal storage.

So it might be possible that nuclear in the end is cheaper than renewables. Personally I'm not sure about that, but I'm not an expert in the field.