I’m not aware of the root cause. I thought they just stopped being able to sleep, and the buildup of whatever junk in the brain eventually caused death.

No, you have the causality reversed. FFI is a prion disease. The insomnia is a consequence of the damage the prion proteins cause. The prions themselves will kill you, regardless of whether you were able to reproduce the biological effects of sleep in a patient. Insomnia is also also caused by other acute brain illnesses, like rabies.

Ah, smarter today than yesterday. Thx