“With any luck, he will appoint a head of ATF to drain the swamp.”
Organization theory predicts that organizations can only get bigger. Typically, when somebody heads an organization they conflate the size and power of the organization to their own importance. To be more important they immediately start growing their empire, collecting new mandates and engaging in mission creep. It would be the unusual man indeed, regardless of his politics, if he accepted a job and then scaled back his organization to its original mandate, or eliminated it entirely.
You are correct. But the BATF seems to be a weird exception. For example, they lobbied Congress to make eliminate 80% of the firearm dealers that they regulated, instead of asking for the manpower to regulate them better.