As others have remarked upthread, political appointees can be purged, but the career civil service people are very, very hard to shift. It would be extremely interesting to see a President attempt to dismantle the civilian personnel system, but I don’t expect it in my lifetime.
When the mushroom clouds disperse, there will be nothing left but cockroaches and government employees with six-figure salaries, huge pension guarantees, and the best health insurance plans.
I would imagine the only way to do it would be by MAJOR budget cuts in DOJ. Under civil service, layoffs are by seniority, so cut the agency by enough that you can fire everybody hired during the Obama presidency.