Why did Paul refuse to impose legalism on his Gentile converts?
Paul refused because law and legalism cannot mix with grace. The allegory of Hagar and Ishmael shows that the bondwoman and her son had to be cast out, not kept nearby. Gentile converts were children of the free woman, so imposing legalism would contradict grace and not work.
Les explains that God set the stage back in Genesis with Ishmael and Isaac being raised for a little while in proximity and then sent apart, so Paul could use them as an allegory for the Law. Just as Abraham did not give Hagar a tent next door or down the line, but sent them clear out into the desert, grace and legalism cannot mix. Paul contended for this and was determined not to give in and command his Gentile converts to embrace any kind of legalism, because it just won't work. In Galatians 4, believers are not children of the bondwoman but of the free, and Galatians 5:1 speaks of liberty, though liberty is not license.