> "
Why would it not work?"
As a thought experiment I'll go with "atom by atom replacement would still be you". But I won't easily go with "therefore we modern humans could actually do it in reality, and replace the parts with fakes, and then with simulated fakes, and then extract the simulation to run elsewhere for the uploading part, and even God couldn't tell the difference" just by asserting all those extra bits to be true. When have we ever made perfect replicas with the inside the same, of anything non-trivial at any scale?
Physical impossibility (it would work but we can't do it) argument, see here for long discussion of how hard the engineering problems of just Neuralink are: https://waitbutwhy.com/2017/04/neuralink.html
Physical "doesn't make sense" argument; we model brains as 80 billion neurons but bodies are not neatly separate organs and neatly separate pieces. The brain isn't separate from the body - nerves are basically brain matter spreading down to your spine, guts, arms, legs. Are 80 billion fake neurons surrounded by layers of scar-tissue reconnections going to be the same? What sense does it make to suggest a simulated sperm could mate with a real egg?