The science is pretty solid and has been for decades. It's just hard convincing investors to bother with something that might take their entire life to see a return on.
I just wanted to point out that there has to be a different reason here. Investors ROUTINELY invest in things that take a lifetime to see a return: they are called Governments. For example, South Korea (or... one of the small countries in that area I forget lol) had a company that wanted to produce steel. But that country had almost none of the resources for producing steel. Private investors thought it was a bad risk, etc. The government ignored 'free market' ideas/neoliberalism, etc. and instead invested in this company and propped it up for years.
That company is now the top, or one of the top companies in the world that produce steel. (edit: I think it is
this company.) Toyota was the same way. The first toyota car was a huge flop. Quality was horrible, few people liked it. The goverment kept propping up toyota well after it would have gone bankrupt on its own, and eventually we have the toyota company that we all know today. This is one of the aruments against neoliberalism. If all investment is only done through private investors, you won't see companies invested in that might take a decade or three to mature and show a return. This is also why the biggest countries push free market/neoliberal ideas.
Unlimited power? Governments would invest in this. We are living in a Peak Oil world, fossil fuels are getting harder and hard to come by. Ever wonder why oil companies drill 5 miles under the ocean for oil? The amount of oil in that reservoir that caused the Gulf spill a few years ago was tiny compared to reservoirs that people found 50+ years ago. And back then people put a pipe in the ground in texas and the oil just shot out of the ground. Now we are fracking for natural gas. Why? Those old oil reservoirs are all long gone. We picked all the low hanging fruit. This is why Iraq gets attacked but not NOrth Korea, even though N Korea kills people in gulags and also has weapons of mass destruction. But no natural resources.
Anyway, just trying to point out, if govenments are not investing in fusion it is because there is no possibility of return at all or some risk is just too high. It has nothing to do with time.