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This is a message about our complacency in just assuming matters are settled and thus don't need to become legislation. If the thing Trump was undoing was a law he'd have to either pass a new law or outright defy it. Neither would necessarily stop him, but it would provide a shock to the people who think anything that's an EO is the current president's prerogative and thus this is the system working as intended. Undoing an EO can be done without any outside support from vulnerable senators and buried in a flood of news stories like this one was, yielding little political cost to the president. We want stuff like this to cost as much as possible.

This isn't to say don't use an EO if legislation isn't feasible, but oftentimes having a public fight to formally establish what the general public already believes is how things should work is a good opportunity to paint your opponents as outside the mainstream.