I think a key flaw in this reasoning is that you extrapolated code needing fewer edits to AI generating entire fusion plants in one shot (in the sense of being instructed once), mostly glossing over the long intermediate period where AI will need significant back and forth to do such things. At the simplest level, it'll need to ask for planning new experiments, experimental results, test runs, etc.

Since these resources are still extracted and allocated by humans, humans will need to be able to take apart what the AI produces, and if we want to scale this capability, we're going to need many more researchers.