If there's a default (I don't think there necessarily has to be one) there has to be somebody who decides what the default is. If most people trust them, that person is either very trustworthy or people just don't care very much.
A web of trust is transitive, meaning that the endorsers are known. It would be trivial to add negative weight to all endorsers of a known-fake paper, and only sightly less trivial to do the same for all endorsers of real papers artificially boosted by such a ring.
