The Potential Impossibility of Subjective Death

You should read Greg Egan's excellent novel Permutation City.

For (1) the multiverse needs to be immensely larger than our universe, by a factor of at least 10^10^6 or so "instances". The exact double exponent depends upon how closely people have to match before it's reasonable to consider them to be essentially the same person. Perhaps on the order of millions of data points is enough, maybe more are needed. Evidence for MWI is nowhere near strong enough to justify this level of granularity in the state space and it doesn't generalize well to space-time quantization so this probably isn't enough. Tegmark's hypothesis would be fine, though.

You don't really need assumption (2). Simulations are not required, all it takes is any nonzero weight (no matter how small) of you not actually dying given each timeline of preceding experience. That includes possibilities like "it was all actually a dream", a physical mock-up of your life, drug-induced states, and no doubt unboundedly many that none of us can imagine.

(3) is definitely required. With (1) there are almost certainly enormous numbers of people essentially identical to you and your experiences, who have the experience of dying and then waking up and you don't know whether you're going to turn out to be one of them, but those are immensely outweighed by those who don't live on.

Sure, those who do live on will think "wow I wasn't expecting this!" and they will remember being you but without (3) they are actually someone else and their future will almost certainly be different from anything you would have experienced had you lived.

What does it mean to "should expect" something, if your identity is transmitted across multiple universes with different ground truths? 

See this comment [LW(p) · GW(p)] which links to a bunch of previous discussions.

This is quite similar to the "swampman" thought experiment (https://en.wikipedia.org/wiki/Donald_Davidson_(philosopher)).

My thoughts: Assuming there is no subjective experience after death (no afterlife or anything), then it is sort of trivial that subjective experience ends at death, so you don't ever experience it.

Now, my read on your argument is that in a sufficiently big universe or multiverse, there will be many "mes" with exactly the same subjective experiences so far, and that whenever one (or a large number) of "mes" die there will be some others who are narrowly saved at the last moment, just as they wheeze their last breath an alien turns up and heals them or whatever. Or they were in a simulation the whole time or similar.

However, it remains the case that before the death there were N copies, and afterwards there were N-1. Its not like you "merged with" or "snapped into" the surviving ones. You are not causally propagating yourself into them. Its just you have accepted a world view where it is possible to likely that there are people arbitrarily similar to you.

My feeling is that its like this analogy. Imagine that in the near future all records of the works of Shakespeare (all of them, including all quotes) are lost forever. But that, it just so happens that by complete coincidence there are pebbles on a beach in another galaxy, that can be read in binary (dark/pale pebbles 1/0) to symbolise the full works of Shakespeare to the letter. Does that make it any less of a loss that the works were lost here on Earth?

So, what's up with my apparent nonexistence in my past? It seems slightly weird that I had some starting point but wouldn't have ending point. Also I'm really confused by, like, subjective time being a thing, if you assume this post is correct description of the universe.

If "I" am dying, but the other supposed instances of "me" are not, then the similarity between us has already been lost. "I" am on my own and will die alone.

Yes, this seems to be the implication from this assumptions. 

This is one of the reasons why I'm very confident that assumption 3. is either wrong, or "not even wrong".

I'm not sure what (if any) action-relevant point you might be making. If this is supposed to make you less concerned about death, I'd point out that the thing to care about is your measure (as in quantum immortality), which is still greatly reduced. 

It's not necessary to bring quantum physics into it. Isomorphic consciousness-structures have the same experience (else they wouldn't be isomorphic, since we make their experience part of them). The me up to the point of waking up tomorrow (or the point of my apparent death) is a such a structure (with no canonical language unfortunately; there are infinitely many that suffice), and so it has an elementary class, the structures that elementarily extend it, in particular that extend its experience past tomorrow morning.

+2 for brevity! A couple more explorations of this idea that I didn't see linked yet. They are more verbose, but in a way I appreciate.

• The mathematical universe: the map that is the territory [LW · GW]. I'd love to meet the author of this. They also wrote the excellent If a tree falls on Sleeping Beauty... [LW · GW]. Sadly they haven't used that account in many years.
• Simulation, Consciousness, Existence (Hans Moravec)

If you want to explore this idea further, I'd love you join you.

I think 1 and 2 are reasonable but I struggle to accept 3. I’m not sure that consciousness transfers that way at all, but there’s no way for anyone to realize this other then the consciousness that was not transferred. https://youtu.be/JMkrrjKf5AE is an excellent video by Jacob Geller on this exact idea.

This is similar to the quantum suicide thought experiment:

https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality

Check out the Max Tegmark references in particular.