This might not be realistic though. For example for a voting app, would you get 2x the voting power for having 2 credentials? Even in QF I’d argue that the match amount shouldn’t equal the personhood score. For example, let’s say we updated the “legacy” trust score to be linear in personhood score. That would require removing the 150% cap, allowing me to get 250% or more in trust bonus if I used all of the verification methods. This number would only go up the more verification methods get added.
So in this system, indeed there would be no incentive to “Sybil”, but that’s because we would just be handing excess power to the user directly.
The math here is actually important. Any system with a cap or tapering will be concave. For example the spreadsheets in the post you linked (where you link to this) are concave (e.g. if the aggregate match was $100k and I was capable of getting $1m cost-of-forgery then I would want to split into 10 passports). And indeed they should be concave: in a world where there are hundreds of verification mechanisms, someone shouldn’t get double the match because they had the tenacity to sign up with 20 verification methods instead of 10 (otherwise you get farming captchas all over again).
Another point to note is that forgeries themselves are not independent events. So it’s unclear if the assumption cost_of_forgery(methods) == sum(cost_of_forgery(methods[i])) is ideal.