It's the sort of problem that is easily prone to extreme optimism extreme pessimism cycles. You get 90% of the way there and then the difficulty
skyrockets. Getting to 95% is as hard as going from 0 to 90%. Getting to where you need to be for it to be mainstream is like 10-20X as hard as making an impressive looking proof of concept.
Space flight is another thing like that. We were supposed to have manned flights to the outer solar system by 2000. We're getting reusable first stages that are actually economical to fly and commercial orbital spacecraft in 2020. That was supposed to be 1980s stuff according to 1950s and 1960s space hype.
Putting someone on the moon? 1969. But that was done with a monstrously expensive finicky one-off architecture that could never be anywhere close to economical, and it was dangerous as hell. Routine trips to the moon that could even approach economic sustainability are at least 10X as hard as Apollo.
I strongly suspect safe, sustainable, and affordable nuclear fission power is yet another one. Getting some atoms to smash and boil some water? Easy! Build some power plants? A little harder. Then you hit the edge of that bath tub curve and the problems (waste, safety, fuel cycle, cost management) multiply and it gets brutally hard really fast.
We should learn to recognize engineering problems that have this kind of "bath tub curve" for difficulty vs ones with gentler learning curves.