If we live in naive MWI, an IBP agent would not care for good reasons, because naive MWI is a “library of babel” where essentially every conceivable thing happens no matter what you do.

Isn't the frequency of amplitude-patterns changes depending on what you do? So an agent can care about that instead of point-states.

In the case of teleportation, I think teleportation-phobic people are mostly making an implicit error of the form “mistakenly modeling situations as though you are a Cartesian Ghost who is observing experiences from outside the universe”, not making a mistake about what their preferences are per se.

Why not both? I can imagine that someone would be persuaded to accept teleportation/uploading if they stopped believing in physical Cartesian Ghost. But it's possible that if you remind them that continuity of experience, like table, is just a description of physical situation and not divinely blessed necessary value, that would be enough to tip the balance toward them valuing carbon or whatever. It's bad to be wrong about Cartesian Ghosts, but it's also bad to think that you don't have a choice about how you value experience.

Analogy: When you’re writing in your personal diary, you’re free to define “table” however you want. But in ordinary English-language discourse, if you call all penguins “tables” you’ll just be wrong. And this fact isn’t changed at all by the fact that “table” lacks a perfectly formal physics-level definition.

You're also free to define "I" however you want in your values. You're only wrong if your definitions imply wrong physical reality. But defining "I" and "experiences" in such a way that you will not experience anything after teleportation is possible without implying anything physically wrong.

You can be wrong about physical reality of teleportation. But even after you figured out that there is no additional physical process going on that kills your soul, except for the change of location, you still can move from "my soul crashes against an asteroid" to "soul-death in my values means sudden change in location" instead of to "my soul remains alive".

It's not like I even expect you specifically to mean "don't liking teleportation is necessary irrational" much. It's just that saying that there should be an actual answer to questions about "I" and "experiences" makes people moral-realist.

I'm asking how physicists in the laboratory know that their observation are sharp-valued and classical?

If we were just talking about word definitions and nothing else, then sure, define “self” however you want. You have the universe’s permission to define yourself into dying as often or as rarely as you’d like, if word definitions alone are what concerns you.

But this post hasn’t been talking about word definitions. It’s been talking about substantive predictive questions like “What’s the very next thing I’m going to see? The other side of the teleporter? Or nothing at all?”

There should be an actual answer to this, at least to the same degree there’s an answer to “When I step through this doorway, will I have another experience? And if so, what will that experience be?”

Why? If "I" is arbitrary definition, then “When I step through this doorway, will I have another experience?" depends on this arbitrary definition and so is also arbitrary.

But I hope the arguments I’ve laid out above make it clear what the right answer has to be: You should anticipate having both experiences.

So you always anticipate all possible experiences, because of multiverse? And if they are weighted, than wouldn't discovering that you are made of mini-yous will change your anticipation even without changing your brain state?

What's the evidence for these "sharp-valued classical observations" being real things?

In particular, a.follower many worlder has to discard unobserved results in the same way as a Copenhagenist—it’s just that they interpret doing so as the unobserved results existing in another branch, rather than being snipped off by collapse.

A many-worlder doesn't have to discard unobserved results - you may care about other branches.

The wrong part is mostly in https://arxiv.org/pdf/1405.7577.pdf, but: indexical probabilities of being a copy are value-laden - seems like the derivation first assumes that branching happens globally and then assumes that you are forbidden to count different instantiations of yourself, that were created by this global process.

"The" was just me being bad in English. What I mean is:

1. There is probably a way to mathematically model true stochasticity. Properly, not as many-worlds.
2. Math being deterministic shouldn't be a problem, because the laws of truly stochastic world are not stochastic themselves.
3. I don't expect any such model to be simpler than many-worlds model. And that's why you shouldn't believe in true stochasticity.
4. If 1 is wrong and it's not possible to mathematically model true stochasticity, then it's even worse and I would question your assertion of true stochasticity being coherent.
5. If you say that mathematical models turn out complex because deterministic math is unnatural language for true stochasticity, then how do you compare them without math? The program that outputs an array is also simpler than the one that outputs one sample from that array.

How would you formulate this axiom?

Ugh, I'm bad at math. Let's say given the space of outcomes O and reality predicate R, the axiom would be $\exists x\in O:R\left(x\right)$.

Carroll's additional assumptions are not relied on by the MWI.