A few things but your concern is totally valid; we are in a bad place if he does not back down.
1. Executive power has limits. E.g. civil litigation and state law are largely outside his jurisdiction.
2. Agents of the government must follow the law, even if it contradicts an exective order. They open themselves to criminal liability if they don't.
3. Congress. Haha just kidding that's only in the timeline where Harambe is still alive.
