If not, why do people speak as if the lack of such a derivation were a point against MWI?
The primary argument in favor of MWI is that it doesn’t require you to postulate additional natural laws other than the basic ones we know for quantum evolution. This argument can pretty easily be criticized on the grounds that yes, MWI does require you to know an additional fact about the universe (the Born rule) before you can actually generate correct predictions.
The primary argument in favor of MWI is that it doesn’t require you to postulate additional natural laws other than the basic ones we know for quantum evolution. This argument can pretty easily be criticized on the grounds that yes, MWI does require you to know an additional fact about the universe (the Born rule) before you can actually generate correct predictions.