I'm not sure the Paris agreement optimises for "far cheaper" but for a maximum temperature increase instead. The consumer can't be expected to individually take one for the team (as it is presumably cheaper in the long run to not deal with a globally failing ecosystem) so through legislation and incentives we'd transition to clean energy, not necessarily by making something cheaper.

Of course, where possible the solution is to just use batteries, but try telling frequent fliers that they'll need to take a train in the future. We're going to need some synthetic fuels where energy density is paramount.

If we assume the price for a carrier nuclear reactor + generator would stay the same. Wouldn't the costs come down simply from basic economies of scale? The costs are high because we've build a few dozen of these ever.