> trust isn’t really transitive

Not sure I agree with this. Sure, trust might drop off pretty quickly (like an inverse square law), but I would still trust a friend of a friend over a complete stranger.

I would also trust a mutual friend over a complete stranger. But that's not the point of the observation: the observation is that "trust" isn't a boolean, but an umbrella term for a wide range of policies that we apply to different principals.

Or in other words: transitive trust is a thing, but it's of a different color than "trust." Attempts to gloss over this in web-of-trust designs have historically not gone well.

I'd argue "friend of a friend" is strong transitivly because it's explicitly chosen by all parties involved.

Trust in a professional relation - a doctor, especially - is actually very strong, because of the professional requirements to be trustworthy, and the protections built into that (being held accountable by an organization and/or lawsuits).

"Family of friend" or "family of professional" isn't necessarily a strong relation for exactly the opposite reason, unless maybe the first-degree contract is vouching for the person.