The web of trust is a really nice idea, but it works badly against that kind of attacks. Just consider that in the real world, most living people (all eight billions) are linked by only six degrees of separation. It really works, for code and for trusted social relations (like "I lend you 100 bucks and you pay me them back when you get your salary") mostly when you know the code author in person.
This is also not a new insight. In the beginning of the naughties, there was a web site named kuro5hin.org, which experiemented with user ratings and trust networks. It turned out impossible to prevent take-overs.
