It’s not even just about the fact that it’s going to wreck those agencies, it also means that there will be substantially less whistle-blowing, and there will be virtually no one working for the government who will raise an alarm or put a stop to anything. When everybody is on board, that creates a substantial amount of power for the executive branch.
What makes it so frightening is that the discussion starts to slide away from the actual functioning of our democratic system and the workings of the executive branch, and starts getting into matters of where power is derived from in a government.
What we have seen is that our Congress is infected by too many friends of fascism, if not fascist themselves. Unless the Democrats have a supermajority in both chambers, Republicans can successfully derail every single thing Congress ever tries to do to reign in an executive branch that’s out of control. Trump was impeached twice, and painfully, obviously guilty both times, and nothing happened because the system has been so fundamentally broken.
Knowing now that Congress can do nothing to stop him, and of course knowing that the court system is captured at this point, Trump will be completely and utterly unafraid of doing anything. The systems in place that would protect us from a renegade executive office will fail to stop him.
Having the entire executive, and every seat in every department filled with loyalists, with nothing in his way that can effectively stop it, is basically a precursor to dictatorship.