Comment by rayiner

You're conflating two very different things. You're correct that civil service reforms sought to ensure employees would be hired based on merit. But that does not mean they were granted "decision-making independence." The point was to have highly qualified people executing the agenda of the elected President--not to allow them to exercise discretion independent of political forces.

In Federalist 70 Hamilton emphasizes that a key feature of the Constitution is "unity" of executive power in the President: https://avalon.law.yale.edu/18th_century/fed70.asp. Hamilton explains that the Constitution expressly rejects a model that had been adopted by several state governments, where the exercise of executive power was subject to the independent check of the executive's subordinates:

> The ingredients which constitute energy in the Executive are, first, unity; secondly, duration; thirdly, an adequate provision for its support; fourthly, competent powers.

> That unity is conducive to energy will not be disputed. Decision, activity, secrecy, and despatch will generally characterize the proceedings of one man in a much more eminent degree than the proceedings of any greater number; and in proportion as the number is increased, these qualities will be diminished.

> This unity may be destroyed in two ways: either by vesting the power in two or more magistrates of equal dignity and authority; or by vesting it ostensibly in one man, subject, in whole or in part, to the control and co-operation of others, in the capacity of counsellors to him.

So the view being espoused here is not a "recent development." Hamilton was explaining back in 1788 the problems with a model where the President was "ostensibly" the head of the administration, but was "subject, in whole or in part, to the control and cooperation" of his theoretical subordinates.

The constitution was understood this way from Hamilton until Myers v. United States in 1926--which held that the President could fire agency heads without Congressional approval because that was necessary to secure his authority to carry out his will as the executive. The Supreme Court only discarded the traditional view of the executive in the 1930s when FDR created the modern administrative state. And what's now labeled "unitary executive theory" is a legal movement that arose in the 1980s to restore the original view of how the executive worked. The new development wasn't the view of executive power, but instead the idea that we should try to restore how things worked prior to the 1930s.