Yes Steve, diagnosing the exact stage of the cancer is impossible. Even post op when the histology report was good, we have seen people on this site with recurrence due to unknown distant mets. The medics can only work with the information they have at the time.
I was T3N0M0, G9, PSA 25, with extra prostatic extension. The view of the medics was that surgery would be pointless on me. Five years later PSA is currently<0.1 . I don't consider myself cured, if someone is not interested in statistics I will used the phrase 'cured to all intents and purposes'.
I think someone who has undetectable PSA more than ten years after surgery, probably is cured. For me statistics suggest I will die of prostate cancer 15 years from today. That date moves forward one day a time, until I get to 70 then the statistics shift to suggest I will die of something other than prostate cancer in less than 15 years time. If my PSA rises above 2.1 the statistics change from 15 years to 5 years.
I don't have any problem living with these probabilities, I accepted I was not immortal at the age of 48, after my dad died (in his 80s).
Had I have had surgery it would improve my statistics slightly, but at the costs of the side effects of surgery, in the opinion of the medics the slight benefit was not worth it, I am inclined to agree with them.