Maybe getting caught causes the island to be shut out and papers automatically invalidated if there aren't sufficient real endorsers.

Unless you can be fooled into trusting a fake endorser, that island might just as well not exist.

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.