it's not that simple. context (timing, culture/politics, market conditions, regulations) is very important.
there's a lot of interest nowadays in "progress studies" which tries to tackle this (or a very similar set of) questions
yes, sure commercial fusion is definitely one of the toughest nuts, but fission based small modular plants are viable, yet it's barely being done. and there are many products and technologies that are economical yet not widespread. (let's say public transport.)
Agree on the complexities. 'Viability' has to be conceived more widely than just a technical sense. Fission a good example, as you suggest. A dead duck in most of the world. As is wide-scale decarbonisation! We know that (a) it is technically feasible, and (b) it is politically infeasible. So decarbonisation is actually not viable, despite appearing technically possible from a narrow perspective.
This also makes commercail fusion very unlikely to happen in this civilisational iteration. It requires large advanced economies with highly functional and specialised supply chains. These conditions won't exist once climate refugees sweep the world in their billions, and wars break out everywhere. If, indeed, given Russia/Ukraine, and the joint determination of China & the US to go to war, we even last that long.
there's a lot of interest nowadays in "progress studies" which tries to tackle this (or a very similar set of) questions
yes, sure commercial fusion is definitely one of the toughest nuts, but fission based small modular plants are viable, yet it's barely being done. and there are many products and technologies that are economical yet not widespread. (let's say public transport.)