That just kicks the can down the road. If our general strategy (aim for exponential growth in all things) remains the same, cheaper power will mean we use more of it. Even if we completely phase out CO₂-releasing energy sources, the waste heat of our industrial processes would eventually dominate the power received from the sun. Even if we solve that… deuterium, like oil, is non-renewable. Doesn't matter how cheap it is: there are only two dozen trillion tonnes of it in the ocean, and once we've run out, we've run out.

IMO, pretty much magic wishful thinking.

I'm enthusiastic about AI. But it's not magic. The main problem here is as Feynmann said, "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled".

In this case the problem is that a fusion reactor is a real, physical machine intensely dependent on the real world in a million ways. From the actual physics of fusion, to manufacturing capabilities, to the capabilities of the various sensors, actuators, processors, etc needed to control the reaction.

You can't magic that up with AI. There's no way for it to figure out that we're subtly wrong about some fact about fusion and that we can get 100X better by just doing things differently. A hypothetical GPT20 would still need to actually perform real physical experiments to gain such knowledge, because it'd be nowhere in our books or the internet for it to ingest it.