Is the severity of foot drop linked to how likely it will be to cure it?

Nope: Outcome depends upon cause, not severity.

Foot drop: Foot drop is usually related to a nerve problem with the lower back, the peroneal part of the sciatic nerve, the peroneal nerve itself or injury to the muscle of the anterior part of the leg. The key to recover is the type and severity of the injury. In general the worse the foot drop the longer it takes to recover.

Yes: If really severe from nerve trauma some cannot reain the use. An AFO brace may be your answer.