Our genome is a machine, from the nucleotides to the packing, to the enzyme activity, to the metabolic flux.
Our bodies are bigger machines made of lots of little machines.
Our minds or conscious egos or "souls" are the neurotransmitter and activation activity of the connectome and all of its cells and synaptic weights and metabolic activity. They're our lived experiences for as long as our brains can function. Minds experience and produce wonderful things.
If you divorce the body from the mind, there is no "person". Just a very complicated machine. A very valuable machine full of parts.
A human body in a vegetative state is not a person. It's a dormant machine. People may have emotional attachment to that vestige, but it is no longer capable of being a person. It is not a person.
We use brain dead humans for organ transplant all the time. If you understand the premise, then it isn't that far-fetched that we might grow vegetative humans in a lab for medical use and research.
Bodies that never have brains can never become persons. They're no different from plants.
My guess is that you're either a dev or an orthopaedic surgeon, well-versed in managing the machinistic aspects of systems, but with little motivation to go beyond them.
There is decent experimental evidence to demonstrate that we are more than gene expression and the machine analogy you insist on is not a good one for understanding biological systems - see work by Michael Levin, as example.
There is a wider paradigmatic shift underway that moves from thinking about parts to processes. This refocus on relations rather than objects is very important and, for biological systems, points to a fundamentally social/collective aspect to their nature.
The machine metaphor also fails when you can no longer explain how the machine works. This is true in many areas of medicine (e.g. anasthesia) and, while we continue to believe (sometimes with enormous zeal) in the concepts that helped us in the past, we cling to them at the cost of building better understanding.
What you say isn't "wrong", but it is too limited to be a useful guide in asking new questions about things like immunotherapy treatments.
You might be surprised at how little of the body still functions without brain function, well, some bits of the brain, including basic homeostasis and immune system function.
Yeah, sure. There are probably going to be only a few tens of thousands "unknown unknowns" side-effects but hey, who cares? We will figure them out, we are out of the stone age cave now!
Clone humans. Cut off their brain stem during development. Turn off cephalization signals for good measure. Scale it up to industrial scale.
Research problems solved.
We'd have every study at our fingertips. We'd have organs and tissue and blood for everyone.
We could possibly even do whole head transplants and cure all non-blood, non-brain cancers.
But we're playing in the sand.