There is a false presumption here in that you are saying Congress specifies the number of positions. They appropriate DOLLARS, not positions, to fill a request to fund a number of positions. My Treasury sets up the accounts, but that does not mean I MUST, by law, spend every dime. Congress doesn't tell me how many people I MUST hire or retain because determining staffing levels is an executive function, not a legislative function. They only tell me how much I can have to accomplish the goals they directed (per an ESA relying upon unconstitutional Section 1531 treaty "authority"). I can decide that I'll get more tortoises by giving Mr. Bundy a contract to increase their numbers with his cows, which he very easily can do, as tortoises depend upon post disturbance plants requiring the nitrates the cows leave behind. But I digress.
Layoff. Contract it out. Make more tortoises and watch the courts squirm using NEPAs citation of "market failure" as supposedly legitimate justification for the emergency powers to protect public health and safety they've been citing as basis for this crap. The market wants to do it. The agency will have to get out of the way and that means they are not needed. Instead I had spent less money more efficiently and eliminated the need for staffing in the process.
Done. And that's just one example.
True. But have you ever worked in government and tried to eliminate a position for which funding was available? Laying someone off from a government job when the budget has been cut is not easy, but doable. You still have a bunch of juggling to do, people bumping according to seniority, etc., but at the end of the day, you can wind up with fewer people. But try doing that when the budget hasn't been cut - it is almost impossible to do. Now if we could change the law to eliminate government unions and civil service protections, that would make a big difference. But without those changes, what you are suggesting would be tied up in the courts until long after the then-current president was out of office.
I am not opposed to the concept - I have just worked in government long enough to know that the idea of doing wholesale layoffs, absent a budget cut to justify it, is right up there with getting an amendment banning abortion passed and ratified - it may be technically possible (i.e., there is a non-zero possibility it could happen), but it is not likely barring some other major changes first.