I think you are suggesting that the PSA can only increase but I don't think you are correct in that assumption if my own experience is anything to go by.
My PSA when I was diagnosed in 2009 was 12.2 ng/ml. I had one year of Zoladex followed by radiotherapy. My first PSA one month after the radiotherapy was 0.07 ng/ml. I continued for another two years on Zoladex which I finished in March 2012 by which time it was <0.04 ng/ml. It has increased since then but hopefully has plateaued around 0.5 ng/ml.
At the time I stopped the Zoladex I was told that the PSA and testosterone would both increase but should level out as seems to be the case.
I hope that you have the same or even better experience.