He has no legal authority to use the Slash and Burn approach you favor. Congress is a separate branch of government is may surprise you to learn and the executive has no power over it for the most part. Retirement plans are established at the time of hire and that is what Congress has approved. The Courts have repeatedly ruled on the right of contract which is inviolable in a country under the Rule of Law.
congress can do anything it wants to federal retirement plans and federal pay with the presidents signature of course.