A couple of questions and a few thoughts.
Left untreated, by whom?
What are you defining treatment as?
Personal experience:
-Just by removing myself from a triggering situations, my PTSD aggravations faded over time. I did fine while I lived a relatively isolated life.
-When I entered the work force and enfered relationships, I could not remove myself completely from triggers, and the triggers were more persistant, and more frequent. My PTSD got worse. Then I sought professional help.
-Treatment sometimes helped, sometimes aggravated; attuned, relational therapy helps, EMDR is practically deadly for me. Some therapists weren't skilled enough to work with me.
Working knowledge of PTSD:
I think it really depends on the person, the situation, the degree of aggravation, and the person's trauma history.