The 25th amendment has been getting a lot of attention lately. Most of that attention is undeserved, unless you expect President Trump to suffer a debilitating stroke. The amendment was written to avoid, in the words of Doug Muir, “The nightmare scenario in 1964 was what if that bullet had hit an inch higher, leaving JFK a vegetable instead of killing him.” So that is what it addresses.
But there was also the example of Woodrow Wilson. What if you had a cabinet of loyalists, refusing to admit that the President is incapitated? Therefore, the amendment allows Congress to appoint “such other body” besides the Cabinet to determine if the President can do their job.
In a future world where the norms have frayed and apathy reigns supreme, that clause looks like it might be dangerous. Have an administration that wants to prevent its work from ever being undone? Have Congress appoint an oligarchic grand council. (Or empower the Joint Chiefs or whatever.) The council, along with the Vice-president, can temporarily remove the president if he or she threatened to do something that the Council disapproved of. Of course, they could not permanently remove the president unless two-thirds of Congress went along, but the threat might be enough. They could certainly get the president out for a few weeks at a time. Of course, the Vice-president is supposed to take over as Acting President, but presumably the council could move against the Veep in turn if he or she did not cooperate.
In that way a regime could perpetuate itself. The threat might not even have to be used: presidents would know that they are figureheads. If you want a science fiction scenario where America preserves the forms of democracy but loses the content, the 25th amendment looks like a possible mechanism, no?
In reality, this could never happen. You would have an outcry, the Veep would not cooperate, the President would declare that he or she is fine, take back office and do whatever it is that the Grand Council was trying to stop them from doing. And the public outcry would be enormous. So there is nothing at all to worry about.