I'd argue that "personhood" is rarely what these things actually care about
This is probably true. Maybe the best use case is actually the opposite of preventing bots: enabling good bots who can't pass CAPTCHAs to access services they need (by paying humans to let them in).
Yeah, I only really mention spam because it's the one obvious use case I could think of and people are already familiar with it. It seems like spam is mostly solved anyway in many domains, so if that's the only thing this proposal can solve it's not much use.