I'm a very skeptical guy.
Things like a self-sustaining colony on Mars are obviously ludicrous to me (in the next century or so at least) because of the incredibly huge cost and the immense amount of new technology that would need to be developed.
Fusion is however not in this category. It is a single technology that needs to be developed, though a very hard one. There's no reason to suppose it cannot be done. We know the physics is correct and we have demonstrated self-sustaining contained reactions.
The most logical reason that fusion "fizzled out" is because the US government, its biggest funder, has repeatedly cut its funding: http://large.stanford.edu/courses/2021/ph241/margraf1/
In almost 70 years of fusion research, humans have spent perhaps $100 billion on it (adjusted for inflation).
Meanwhile the top four US technology companies alone spend over $25 billion a year on software and hardware research, and humanity spends $2 trillion a year on warfare.
Put another way, the entire investment in fusion over two generations is less than what humans spend on warfare in any given three week period.
The potential value of fusion is so great that it's worth investing trillions into it even if success is not assured.
The basic process intrinsically generates a tiny amount of helium, a completely harmless and inert substance that is neither chemically nor radiologically toxic. Yes, there will be additional industrial crap involved in building the factory, but the actual process is heavy water -> helium and energy.
It does this from seawater, and it doesn't even consume the water, just a tiny portion of it which contains heavy hydrogen.
And because these reactions are only sustained because of massive magnetic fields, there seems to be no conceivable possibility for any disaster beyond a local "fizzle" type explosion. A tidal wave hits the plant? The magnetism stops instantly, and in microseconds the fusion stops.
We know an awful lot about how hydrogen fusion works by now, and we know about as sure as we know any science that it cannot be a self-sustaining process until you have masses of hydrogen that are larger than Jupiter!
Cheap, copious energy which generates far less pollution than any other energy generation method, which runs off sea water and generates hydrogen, and doesn't have the multiple destructive failure mechanisms that fusion is not just "nice to have", it is literally the only hope we have to combatting the climate disaster.
