The truth is that people both over- and under-estimate what can happen in the future. You can't use simple-minded heuristics either way; technoutopianism is at least as dangerous as blind opposition to change.
Not saying that fusion power on earth is viable (I personally think that it is), but it's disingenuous to compare it to perpetual motion.
At this point we just don't know whether fusion is like airplanes or whether it's like perpetual motion. Unless you are proposing building an actual star, the existence of stars tells us approximately nothing about whether a fusion-based power plant is economically achievable in our lifetimes.
I think it's an interesting idea and I'd love to see it happen. But I think it's important to be realistic in our discussion of it.
That's not correct at all. The simple-minded heuristic is along the lines of "technology is bad" or "this new thing will never work".
But the dream of fusion as "too cheap to meter" has been going on for 70 years. The comment you're replying to wasn't talking in general terms, but had specific critique of both the long track record and the practical utility of the product they're shooting for.
It might work and it might not; I have no idea. But your attacking a straw man won't help it get there.
ITER is the Samuel Langley of the present day.
Hacker News comment in late 19th century: "They are forever making flying machines have a little more lift, on the verge of break-even, on track to enabling free flight without nasty immediate crashes. But it is always a con.... The only reliable product of flying machine research is dishonesty."