We all know

That Joseph isn't just frail right? He is sick and like a terminally ill patient. Even if he becomes the family head he won't last long and die. Also the concern of continuing the Bayezid bloodline. They tried to find a cure or make him healthy with Elven medicine or tea I think (can't fully remember) but it still doesn't work. His father knows its a cruel choice to give him hope, a fighting chance to be a family head for fairness sake but still in the end he didn't win. Though I think he tried to give him something to do to spend his remaining days or years peacefully or maybe he got exiled idr but we all know how he is. He is stubborn, he chose the way he will depart their world and picked a side. He did not surrender to the sickness plaguing him.