> sunk cost fallacy

...doesn't really make sense here.

Let's say it takes 20 years and 5+ institutions to fully explore a single avenue of research.

If you're 15 years into that research and you think there is a chance that you'll reach a breakthrough in 5 years, you're much more inclined to want your (and the other 4+ institutions) to pursue it for 5 more years rather than starting over.

That's not sunk-cost fallacy. It's more like "a bird in the hand is worth two in the bush" or perhaps confirmation bias (the inertia the scientists felt to discard their conclusions about the causes and treatment pathways for Alz).