I understand your point, but why the separation of coin vs. non-coin tech? Meaning that the only real guarantee of cryptocurrency is prevention of double-spend rather than a confirmed unique identity?
Blockchains can extend the guarantees of the blockchain only to concepts that can be completely represented on-chain, which basically meant just coins. They don't have to interface with the real world so aren't subject issues at the boundary. At least that's one way of looking at it. As soon as you start trying to represent things that actually exist in the real world authoritatively, it breaks down - because the authoritative representation is actually off chain, i.e reality. In this case your identity is the authoritative representation, not the key you present.
This is really well said. I’ve been trying to articulate this point for a long time. I think the boundary is between things that are temporal or bounded by real time and space vs things that are time independent and state based. Oracles always suffer from this problem even if the authoritative representation is another digital thing. For example the whole NFT url swapping thing. Similarly this comes up in version control a lot when your code depends on things that are unversioned, the commit state may be idempotent but the application is not.