Deciphering a captcha or staring at a camera each time you want to post are real annoyances and such a site probably wouldn't get much traction. But if the right infrastructure were in place, a lot of us would be happy to participate in a community where we're confident that everyone there is an actual person, not a spambot or a sock puppet.
The right infrastructure here could a tall order, but I don't think this is an impossible problem.
* The iPhone stores your biometric data in its Secure Enclave
* The Secure Enclave also holds the cryptographic private keys
* Every action you do against the server requires your phone to add a cryptographic signature, which can only be written when the iPhone verifies you via FaceID
Apple could act as the Certificate Authority letting websites know that this signature aligns with an iPhone user. Apple may not need to let the website know exactly who performed the action, but just say that Apple verifies that this is a real person making the action.
Maybe the internet sends you a DNA sequence and you have to inject it, translate it and then image the folded protein (computers are ok at protein folding but not great yet)
You can already pay people to do things for you, using their "online identity". This will just continue that.