E.g. if some process in steel manufacturing is improved to require less power, you might think, yay, there's less power needed so less CO2 released. Good for mitigating global warming. But what might actually happen instead is that it becomes feasible to make more things out of steel because it is cheaper than before, so more energy is used on steel production in total, even though per unit it is more efficient.
The same thing could happen with fusion energy, but in ways we can barely imagine now. What high-power workloads are not even considered now that would be cheap enough with fusion power?
Your explanation reminds of the common example that increased car safety leads to more speeding. And somehow arguing that increased safety is futile.
But people like having more steel and like being able to go faster at the same level of safety.
