LHP2017,
You are in the majority. We can guess, but no one can accurately tell what someone's potential is. We only know that very few, if any, actually reach their full potential. and no one has ever come close to reaching their full potential without working hard. Truth is if you have some ability to start with, it can be very rewarding in trying to reach your full potential. There are things you can control (effort, knowledge, etc.) and other things you can't control (growth, body type, etc.)
Also here is something else to think about. Velocity spikes can and do happen at different stages. There are cases where pitchers have gained lots of velocity long after they left HS. Once again due to technique, maturity, growth, improved work ethic, knowledge, etc.
We can talk about those that throw 95-100 mph, but they are unique. Truth is those that can throw upper 70s to 80 mph, have a real good arm. Sure it might not be enough to get a lot of interest from top colleges or pro scouts, but the vast amount of 16-17 year olds can't throw a baseball 80 mph and most of them never will be able to. Some of those are star HS players. So if you can throw 80 you're pretty darn good. And that is not your ceiling!
Just my theory, but I believe genetics has a lot to do with throwing ability. I have just seen too many families where everyone including the females have a strong arm. So if dad had a good arm, chances are his sons have a good arm. Nothing scientific about that, I'm sure there are exceptions, but I have become a believer.