I don't think that's a good metric. Societies that aren't open to mass immigration can have negligible numbers of immigrants regardless of the quality of life their members enjoy. Japan is the prime example.
Moreover, in the very worst places, emigration can be negligible because people can be too poor to pay for the ticket to move anywhere, or prohibited to leave.
But "given perfect knowledge of all market prices and individual preferences at every time and place, as well as unlimited computing power", you could predict how people would choose if they were not faced with legal and moving-cost barriers - e.g. imagine a philanthropist willing to pay the moving costs. So your objection to this metric seems to be a surmountable one, in principle, assuming perfect knowledge etc. The main remaining barrier to migration may be sentimental attachment - but given perfect knowledge etc. one could predict how the ch...
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