I will consider Heidi fully leashed trained when I do not have to use the prong collar any longer. We are getting to the point where she is walking very nicely with me and Buddy without pulling. She likes to walk ahead of us a lot of the time and that's fine with me. Unless I have reason to keep them on a short leash in a heel, they are free to walk whereever they want as long as they don't pull me. So, we are getting closer, but she is not there yet.
Buddy had the need for a prong when he was young too, but only for a while. I can walk him on a flat collar and he stays right by my side for the most part. When he sees something interesting (cat, squirrel, deer, etc), and his ears perk up...all I have to do is give him a soft tug, tell him "not for you" and we are back on track.
I agree with you that the tool is to be used to get to an ultimate goal and further agree that they are not trained completely if the tool is still needed. The prong should be a temporary training tool, not a forever walking collar, IMO.