Nobody has derived the Born rule, though I think some have managed to argue that it is the only rule that makes sense? (I’m not sure how successful they were). I think people may count it against mwi because of either simple double standards or because it’s more obvious as an assumption since it’s the only one MWI needs to make. (In other theories the rule may be hidden in with the other stuff like collapse, so it doesn’t seem like a single assumption but a natural part of the theory. Since MWI is so lean, the assumed rule may be more noticeable, especially to people who are seeing it from the other side of the fence.)
Nobody has derived the Born rule, though I think some have managed to argue that it is the only rule that makes sense? (I’m not sure how successful they were). I think people may count it against mwi because of either simple double standards or because it’s more obvious as an assumption since it’s the only one MWI needs to make. (In other theories the rule may be hidden in with the other stuff like collapse, so it doesn’t seem like a single assumption but a natural part of the theory. Since MWI is so lean, the assumed rule may be more noticeable, especially to people who are seeing it from the other side of the fence.)