> a peer knows that a signature is valid because it can chain it back to a pre-established root of trust, rather than having to establish a new degree of trust in a previously unknown party.
So the apt binary on your system comes with the public keys of the Debian packagers and then verifies that packages are signed by them, or by someone else whose keys you've chosen to add for a third party repository. They are the pre-established root of trust. What is obtained by further centralization? It's just useless indirection; all they can do is certify the packages the Debian maintainers submit, which is the same thing that happens when they sign them directly and include their own keys with the package management system instead of the central authority's, except that now there isn't a central authority to compromise everyone at once or otherwise introduce additional complexity and attack surface.
> PKIs like the Web PKI mediate this by having multiple central authorities (each issuing CA) and forcing them to engage in cryptographically verifiable audibility schemes that keep them honest (certificate transparency).
Web PKI is the worst of both worlds omnishambles. You have multiple independent single points of failure. Compromising any of them allows you to sign anything. Its only redeeming quality is that the CAs have to compete with each other and CAA records nominally allow you to exclude CAs you don't use from issuing certificates for your own domain, but end users can't exclude CAs they don't trust themselves, most domain owners don't even use CAA records and a compromised CA could ignore the CAA record and issue a certificate for any domain regardless.
> It's worth noting that the kind of "small trusted keyring" topology used by Debian, Arch, etc. is a form of centralized signing. It's just an ad-hoc one.
Only it isn't really centralized at all. Each package manager uses its own independent root of trust. The user can not only choose a distribution (apt signed by Debian vs. apt signed by Ubuntu), they can use different package management systems on the same distribution (apt, flatpak, snap, etc.) and can add third party repositories with their own signing keys. One user can use the amdgpu driver which is signed by their distribution and not trust the ones distributed directly by AMD, another can add the vendor's third party repository to get the bleeding edge ones.
This works extremely well. There are plenty of large trustworthy repositories like the official ones of the major distributions for grandma to feel safe in using, but no one is required to trust any specific one nor are people who know what they're doing or have a higher risk tolerance inhibited from using alternate sources or experimental software.