I think it's important to remember that he could care less if nothing works so when say replace, what he might actually do is just remove.
He could cripple the federal government and just not replace people.
Eventually, they will get replaced but he could do a ton of damage by removing any politically appointed officials, auctioning off those positions to business interests, and having them fire everyone they can (or will, really, becuase I think you'd be going to court over this).