Point by point, Paul builds his case to the Christians in Galatia; that, having received life in Christ by faith, they must not revert to an attempt to achieve God’s favor by keeping the law. And his next point is that the promise of God to Abraham and to Christ His Descendant, came 430 years before the law. It is still in effect and irrevocable. As even human covenants remain, it is subject to neither amendment nor being put aside.
And further, God cannot break His promise; therefore, the giving of the law, (again, 430 years after the promise) in no way cancels God’s promise to Abraham. God’s gracious promise to Abraham remains just that– the promise of an Eternal inheritance, a great blessing made available to all nations of the earth. Were the inheritance to suddenly require achievement by the perfect keeping of the law, it would cease to be only the gracious promise of God, accepted by faith. But the promise of God, given to Abraham, remains God’s Promise, irrevocable, without amendment, and not subject to being cancelled.