I will treat my gelding with PPID the rest of his life, however long that may be. He was diagnosed in October and is doing fantastic on medications. His coat is shedding out (he was clipped), he is loosing weight and has more energy. He is 19 now.
PPID is a condition that requires long term management but it is manageable. I don’t think you are wrong in your thinking though and I would treat him while he is comfortable and then let him go, especially a horse that has other chronic health issues.