I don't have much to say about this proposed policy that would prevent British citizens bringing foreign-born spouses or children into the UK on family visas unless they are making over the median wage. Just that, for anyone familiar with people who have become married whilst overseas, and who then move home to find work, you're basically telling them that they cannot live with their spouses and children permanently in their own home country until they earn more than 50% of the British population.
These policies are usually suggested on the Goldilocks principle - a horribly excessive policy is suggested in order to get people to accept a less strict policy. Therefore it seems likely that if any such policy is implemented, it will set the bar somewhat lower.
However, even such a "just right" version of this policy would be a failure because the British government can no longer restrict immigration from mainland Europe. Such a policy would, anyway, only prevent people entering the country legally, without having any effect on illegal immigration.