I agree with you that greater freedom of movement would lead to migration to nations with better job prospects. I don't think that's a bad thing though. Within England, different cities are better or worse off in terms of their economies. Cities doing better may attract people from distant cities within England, though this has a limit as the opportunity would have to be great enough for someone to leave their home. Overall, England is better off because of this. That same concept and logic could work for the whole world. It would probably lessen (though not eliminate) economic disparity between nations--but I think that's a good thing. The UK population has grown as the EU has expanded but by small measures that are very manageable. This is obviously a controversial topic with a variety of valid opinions with England going forward with Brexit, but as you may imagine I was pro-Remain. (I did my MA at UEA in Norwich a few years back).