In my long career in both public and private sector I observed that many people can be removed from a private company - even some who are considered "irreplaceable" - and within a few months nobody can even remember what they did. Actually it happens all the time.
In my 8 years with DoD (3 years military) I saw so much wasted time and effort it finally made me decide to quit. My whole ORGANIZATION could have been eliminated without significant trouble, if related organizations had decent managers.
As a general proposition, government employees never quit. Where would they go? No real employers offer the combination of pay, benefits, and absurdly-early retirement that the Government offers, and most private sector employers are very leery of offering a responsible job to an ex-government person. The chances of him being a total slug are just too great. Same for Military officers. Generally worthless in the real world.
If 25% of the entire Federal workforce resigned, it would take about a year for everything to get back to normal. In most agencies the main thing impeding recovery would be other employees dragging their feet, to show resentment at having to do a little extra work. But this is all bullshit. As said above, where would they go? Nowhere.