Why would anyone go to Kent C Dobbs to verify that he controls the key instead of going to Joe R Badguy or whoever is listed as the author of the malicious package?

There's no secure list of "good people" and there's no secure list that provides a mapping of who should be signing each package. Especially for things maintained by multiple people, I wouldn't have (and shouldn't have to have) any idea of which particular people are the proper signers.