Problem 1. Long Iteration Cycles in Cloning
The first real problem with transplantation onto a non-conscious clone is that each iteration of growing such a clone takes many years. A billion people could die while we complete a single iteration. Moreover, the proposed threshold of 11 years is debatable — the true minimum developmental age may be closer to 18 years.
Problem 2. The Immunity Problem in Allotransplantation
Allotransplantation solves the problem of the absence of a clone, but introduces the problem of immune rejection. This may potentially be addressed through bone marrow hybridization.
Problem 3. Poor Scalability of Allotransplantation
Allotransplantation does not scale well.
Problem 4. The Complexity of Brain Transplantation
Transplanting the brain itself is significantly more difficult than transplanting the head. The brain is extremely soft — almost like panna cotta. In addition, there is the question of the brainstem: there is no reason to assume it is not critically important. Rehabilitation after a brain transplant would also require a far greater degree of neural relearning and adaptation.
Solution 1
Head transplantation may be preferable because it preserves a larger portion of the spinal cord and simplifies rehabilitation. There are already studies on spinal cord fusion, and the research groups involved continue advancing this work. However, the immunity problem would still remain.
Solution 2
There are two ways to improve scalability:
Transplant not onto a full body, but onto a young biological body core without a head and limbs (for example, around 4 years old). This biological core would provide blood circulation, immunity, and essential physiological functions.
Use a robotic body for the remaining physical structure and motor system.
Remaining Problem
The remaining issue is the cerebral vasculature and the health of the blood-brain barrier (BBB). The encouraging aspect is that aging becomes localized to essentially one organ, which is far easier to address than systemic aging. The downside is that cerebral blood vessels are not adapted to a new heart, and altered pulsation dynamics could accelerate vascular damage.
The price is usually a function of scale, so the more people will take it, the cheaper it will be