It’s not happening now, so to that extent it is science fiction. It depends on what you mean by ‘just science fiction’. Hand transplants were science fiction before they were a reality.
Science fiction is really about when we use fiction as a construct to imagine possible futures and help us process them. The ethical issues that arise from robots have been prefaced in science fiction before.
https://www.youtube.com/embed/6zHDdIKPbwA?wmode=opaque
In terms of the question, and regarding brain transplants, it’s the same as hand transplants 50 years ago. We don’t currently have the technical expertise to do a brain transplant. Essentially because the wiring is so complicated.
With things like life support, keeping the supply of oxygen to the brain is less of a problem. When you’re on life support effectively the brain doesn’t need the body in that sense - it’s oxygen are supplied by the heart and lung machine.
https://www.youtube.com/embed/YGt8c_IVwiM?wmode=opaque
The tricky bit is the wiring. And there’s two elements to that. Firstly, you have to connect up millions and millions of connections that connect the brain to the body. So there’s a wiring problem of that nature. Then there’s a further problem that in your development your brain and your body have developed in parallel. So you’ve developed in your brain a representation of your body, both in terms of your senses and in terms of how you move your body. So even if you were able to connect up my brain into your body, it’s not clear that it would operate because my brain doesn’t know about your body. It doesn’t have a model of it.
https://www.youtube.com/embed/m8U4NMEMSZc?wmode=opaque
It’s as if you took the control computer of the London Underground and connected it into the New York Subway. Could you connect the London Underground computer to all the signals on the New York Subway? Yes. Would that work for all the trains? Probably not because the London Underground control computer knows nothing about the New York Subway trains and tracks.
https://www.youtube.com/embed/U9Tldw1c0K0?wmode=opaque
So you have a problem with connections and you have the problem that the brain was developed to connect to a specific body. So is it science fiction? Yes. Is it just science fiction? By that do you mean could it ever happen? No, it’s not impossible. Am I sceptical about how effective it could be? Yes, because of the reasons I’ve outlined.