>> First, the Manhattan Project itself was sui generis. There is very little reason to believe that this model does, in fact, generalize widely outside of the basic and applied sciences.

No. It was a basic question of practical physics. It was an effort to translate theoretical knowledge already proven in laboratories into a working product at scale. The innovations were definitely costly, but essentially rather incremental and straightforward. That was much of the reason it needed to be done in a hurry: eventually someone would do it.

Fusion power is in the same place. The theory works in the lab, we need to translate that into a working model at practical scales. Eventually someone will crack the problem. A Manhattan-style push would accelerate that date, hopefully to within our lifetimes.