I am not defending a specific implementation of biometricx + blockchain, like Worldcoin, merely pointing out that biometrics + blockchain is not inherently doomed for failure.

For a crude hypothetical example, suppose instead of iris scanning tattoos were used. Just for some artificial fluff consider some future human civilization, after the collapse of growth oriented racketeering, which was followed by naive universal basic income (resulting in wealth concentrating in land owners, since they can ratchet up rents to match any basic income), which in turn also collapsed. So in response the World Cadastre Coin was created: it was agreed that regardless of religion or atheistic beliefs, almost surely no-one believes humans created the earth, the sunlight that falls on it, the plants we eat etc. So this hypothetical future society agrees that since there was no human creator of the earth, that each parcel of earth, sea, land etc is either marked for wild nature or for human utilization. Those parts of the earth earmarked for human exploitation were agreed to be rented out to the highest bidder, with all rents instantaneously redistributed on a per capita basis. So everyone gets the average rent. By then humans had settled on due process and rule of law on a blockchain governed by formal verification, quickly resulting in policies with provable properties getting more attention than policies with vacuously proclaimed properties. Since the average rent system has the attractive property that it is affordable by definition, and doesn't require income taxation it quickly became the population preference. If one wants to live in a place that costs more than the average rent, or economically exploit land, territory, mines etc. worth more than average you'll have to work for the additional income to afford it. Those who are satisfied with less than average can save up money for when they decide they are ready to try their own entrepreneurial idea.

In their timeline's past, it was never forgotten how big tech had turned out complicit with the CCP (which collapsed due to provable impossibility of socialized healthcare, if one also takes into account the future generations). So it was decided that to establish sybil-free tokens, one couldn't rely on iris scans, retina scans etc for which a complicated device was required. Instead uniformly located tattoos were selected, on the forehead. Everyone could generate a unique number and have it tattooed on their forehead. Anyone has the ability to read a tattoo when given such opportunity. The blockchain associates a cryptographic receiving address per personal tattoo number. On that address you receive one average rent second per second, or accumulating over the span of a month one average rent month per month. (Underlyingly the blockchain actually uses the elapsed time whenever you make transactions,...). If one somehow loses their private key (stolen computer, etc.), all one has to do is generate a new keypair, report the issue (have some pre-indicated friends confirm the issue on the blockchain), upon which the blockchain as a group rolls provably random dice to select random citizens in a relatively big neighborhood around you. These random citizens will function as proctors: they inspect if you have the relevant number tattooed on your forehead, and if the proctors upon returning to their respective homes are in agreement the blockchain will associate your new address public key which you shared with the proctors to the abstract citizen indicated by your forehead tattoo number.

A sane society would have keypairs expire after some time so that people must repeat this process, so that next of kin aren't receiving average rents for already deceased family members for years on end.

There would be no incentive for a criminal to "steal" your tattoo number and modify his tattoo to look like yours: the same average rent is awarded to him as to you. The only incentive would be to receive your average rent on top of his own, but that wouldn't last long as upon deassociation of your private key with your tattoo number you would promptly arrange for a reidentification so you can start receiving the average rent again. It would be a real petty criminal to repeat tattoo his own face for what, a few hours or days of rent? And he'd get caught eventually.