Yeah the main question is how to preserve privacy. Seemingly the best way is to just have multiple verifier services.
But the thing is, their ability to truly identify a human accurately would still be inferior to a full Sybil resistant network right? People could share fake credentials and scam the verifiers. I guess multiple verifiers would compete on who could verify the best, but still. The meta game could incentivize the verifiers to allow a few exceptions here and there.
A decentralized solution like the two I linked above (maybe there are others too) might be better