Anonymity is the keystone of free speech. There is no freedom of speech without the freedom to speak anonymously.
Sybil attacks are a consequence of anonymity combined with low cost of identity creation, and inability to distinguish between different identities.
The fix for Sybil attacks generally involves making identity creation more expensive, or providing a means for distinguishing more valuable identities. On HN, for instance, the karma score is one such mechanism.
These systems can work without removing anonymity.
And I never said anonymity was easy or without trade-offs.