I really do hope ASI gets us to gradual replacement-based uploading. Having something as glorious as sapience trapped in a delicate and temporary bag of flesh kinda sucks.
'The President of Global Enterprises Inc. is today 150 years old and currently holds the record for the longest serving president in the Company's history'.
A cause for celebration?
For me, personally, I don't see that being the case for many hundreds or even many thousands of years at least. I'm nearly 30 and I feel like I've barely lived a blink of an eye.
I agree it would be weird at first, but after the first thousand years or so, it would probably become common enough that people wouldn't mind, and the taboo would disappear. Weird, maybe, but at least people will get to live their lives to the fullest and at their own pace.
So yeah not sure I like the idea.
You can likely replace the large "functional groups" of neurons instead, with the group size threshold being the maximum that doesn't meaningfully affect our consciousness. This might well be many billions of neurons at a time.
Scan and copy never made sense to begin with, as you point out. Not sure why it's the default when people think of mind uploading.
When you replace a heart with a pump, you don't get a human heart. When you replace a kidney with a dialysis machine, you doon't get a human kidney. Why expect that when you replace neurons with simulations you get a human brain or a human consciousness, or when you've replaced everything, a human?
Biological replacement, your body growing more new neurons, maybe, but it won't be mind uploading. And it won't get you you-at-age-twenty back.
The examples you listed are current technologies that don't remotely approach the primary nor secondary functions of the originals.
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?
I – and I'd wager most people – would be fine with some level of alteration to their being if it meant they wouldn't literally die, and that they would get to spend some more time with their loved ones. Change is a part of life, and this would just be another one.
There is no technology I can see which would convince me that a mechanical replacement hand was my hand and my hand was not literally dead. Only Prof. Michael Levin's work of regrowing real limbs might do that - see https://www.popsci.com/body-electrician-whos-rewiring-bodies... and he has given many video talks, I saw one of him presenting at Facebook about his work triggering regrowing of frog's legs, but there appear to be many more videos of him - TED and Lex Fridman interviews.