Generally speaking, jobs as such are filled internally when anyone has already had experience in that position, as is the case with Cook.
I don't like to speculate too much, but given the man's obviously degrading health, retirement may be the best choice for the sake of his own well being, and any professional acting solely on the interests and his health and well being will say the same thing if his condition has deteriorated more than suspected. Liver transplants have come a long way, but people with liver transplants have a much lower lifespan and almost always take immosupressents for the rest of their life. Given he has already had major illnesses, repressing his immune system has the potential to cause significant harm and a much higher probability of a transplant recipient without previous health issues such as his. Also, infectious and chronic disease development will happen at a faster and higher rate in people on drugs to decrease immune response. Without the immunosupressents however, virtually all transplant patients will die...it's a double edged sword.
I really hope this is not liver related as if it has anything to do with the liver, it is beyond serious. Unfortunately, given he's had a transplant, some type of degenerative liver disease and cancer, ANY health issues he has will have the potential to have devastating effects. The man needs time to relax, reduce stress, and get back to living as best he can. Most likely, the stress of his position is not doing him any good and I am actually glad to hear he is taking a break and getting himself together health wise.
