I'll respond to that, but it doesn't really matter to my basic point: that nuclear fusion startups are all, without exception, in a research phase with no clear path to a commercial product. In other words, investors are currently funding basic research, whether they realize it or not.
But to answer your question:
> How could anyone know that?
By studying the basic physics and engineering involved. The same way we know we aren't going to be doing interstellar travel in the next 50 years.
> I’m guessing the same could have been said about what LLMs can do today, 20 years ago.
It's not comparable. LLMs didn't require dealing with a plasma ten times hotter than the core of the Sun, without the benefit of the Sun's gravitational pressure and density. Commercially viable nuclear fusion power is so far beyond the capabilities of current, and possibly even future, engineering that it's not even funny.
A better comparison would be if there were a bunch of interstellar travel startups, giving dates for when they plan to reach Proxima Centauri. Why don't those exist? Because nobody would believe them, for the kinds of reasons I've already mentioned. But with fusion power, we could theoretically do that here on Earth, leading gullible investors to say, "So you're saying there's a chance!"
> Predicting innovation is inaccurate enough to be silly.
Innovation is what happens after the basic research has pointed the way. We're not at that stage with fusion power.
> Investors are betting that someone figures it out on a shorter time horizon.
Investors who haven't done good due diligence, or who foolishly trusted founders, are betting that.
A couple of references:
"Fusion power unlikely to become competitive" - https://www.nature.com/articles/s41560-026-02022-9 : "Policymakers should not rely on, or fund, fusion power as a core pillar of future clean energy systems unless designs with different characteristics are developed."
"Notes on the Recent Hype about Imminence of Commercial Fusion Energy" - https://www.belfercenter.org/research-analysis/notes-recent-... : "I think it’s fair to say that the scientific and technological problems that must be solved on the road to a practical fusion reactor—beyond energy breakeven in sustained operation—are not much easier than the breakeven problem itself, which is still unsolved after 70 years of costly international effort."
"Fusion reactors: Not what they’re cracked up to be" -
https://thebulletin.org/2017/04/fusion-reactors-not-what-the...