They don't know if it is evolving or progressive. SFN is something they either can or can't find a cause for. If they find a cause, it can be treatable or stopped. If they can't, the only treatment is pain meds and watching and waiting. The biopsy tells that their is damage. I don't see how they would be able to tell there is ongoing damage unless under the microscope they saw the "thing" causing the damage in which case they would have a cause.
I am reliving the desperation and sadness I felt when my first neuromuscular dr gave me my diagnosis in a miserable hopeless way. Don't give up hope.