Fission could and is a sustainable way to have green energy forever. No need to transition at all.
Fusion is unlikely to be cheaper anytime soon, even if somebody could build a plant that makes positive energy.
Fusion is unlikely to be cheaper anytime soon, even if somebody could build a plant that makes positive energy.
In addition, the materials you need for a nuclear reactor are pretty simply and can be done with 1960s technology, while the materials for a fusion reactor are literally unknown.
So all the assumptions about fusion are just wrong, the increase in energy density of fusion over fission is pointless, unless maybe we want to travel do another star system.