I agree with the general line of reasoning you're putting forth here, and you make some interesting points, but I think you're overconfident in your conclusion and I have a few areas where I diverge.
It's at least plausible that an AGI directly descended from LLMs would be human-ish; close to the human configuration in mind-space. However, even if human-ish, it's not human. We currently don't have any way to know how durable our hypothetical AGI's values are; the social axioms that are wired deeply into our neural architecture might be incidental to an AGI, and easily optimized away or abandoned.
I think folks making claims like "P(doom) = 90%" (e.g. EY) don't take this line of reasoning seriously enough. But I don't think it gets us to P(doom) < 10%.
Not least because even if we guarantee it's a direct copy of a human, I'm still not confident that things go well if we ascend the median human to AGI-hood. A replicable, self-modifiable intelligence could quickly amplify itself to super-human levels, and most humans would not do great with god-like powers. So there are a bunch of "non-extinction yet extremely dystopian" world-states possible even if we somehow guarantee that the AGI is initially perfectly human.
> There is every reason to expect a human-derived AGI of beyond-human scale will be able to rationalize killing its enemies.
My shred of hope here is that alignment research will allow us to actually engage in mind-sculpting, such that we can build a system that inhabits a stable attractor in mind-state that is broadly compatible with human values, and yet doesn't have a lot of the foibles of humans. Essentially an avatar of our best selves, rather than an entity that represents the mid-point of the distribution of our observed behaviors.
But I agree that what you describe here is a likely outcome if we don't explicitly design against it.