> Currently, every Tor client uses a trusted group of ~7 "directory servers" to assign them to a circuit
The directory servers don't assign clients to a circuit, they publish a directory of eligible nodes from which all clients can use to select a circuit. The distinction is smaller than it might seem since control of the list ultimately means control of the nodes in the circuit and while, in theory detecting directories giving different nodes different views should be easy I don't think any infrastructure exists for that. I'm just being pedantic on this point because some people might be confused here.
> However, we do consider sybil attacks. Addressing them is the primary purpose of the TorPath assignment protocol. It ensures that, even if an adversary controls over 50% of the relays
Definitional disagreement, I guess. I don't consider this to be sybil resistance. It's very easy to describe things which are secure when a majority is honest, to call something sybil resistant I would want to have an argument as to how its costly (or impossible) for the attacker to achieve the required threshold.
Yes, the odds are low at 50%, but they're certantly not at 99%. You have an amplification, but I didn't see any argument why 99% is substantially harder than 50% (or 1%). Maybe it is if you invoke back in centralized trusted authorities (well, ignoring botnets), and indeed— this could be an advance over tor— but I think some careful though should be given to audibility of the authorities, since I think you lose that if you hope to hide the participation in the network from the public.