It is easy to create a crypto currency that is not vulnerable to sybil attacks; define a central authority. The source of the sybil weekness is not being a crypto-currency, but rather being a peer to peer system.
Re:Ethereum Proof of Stake FAQ
by tlrobinson on Sunday August 20, 2017 @08:14AM
(#15058096)
A central authority isn't really required (depending on your definition of "central authority"), just a connection to at least one semi-trusted node. For example, Blockstream Satellite.
Of course, to economists the concept of a "Sybil attack" is nonsense. Imagine if tomorrow the US Congress voted to give every citizen a billion dollars. Would this be a Sybil attack on the US dollar? Or would it be democracy at work? There's no difference. At the end of the day the majority (of the authority) sets the rules and does literally decide what happens.
[1] https://en.bitcoin.it/wiki/Weaknesses#Sybil_attack