Just a flurry of H₂O

@BridleTimeout

noone should ever have supreme authority over an entire country and all its assets

If I had the choice between King David of ancient Israel, suitably educated on modern conditions, and duly-elected President Trump (never mind Hilary or Bernie or whoever), I don't think it would be a very difficult choice. Likewise King Arthur of pre-medieval England, depending on what legends you consider authoritative, could be pretty solid. The problem is that then you have to deal with the son or grandson (or great-grandson) not doing so hot, or alternatively a succession crisis. (See King Rehoboam, Mordred, etc.) If there's no real chance of an inept, selfish, lazy, or tyrannical successor, then getting a single good ruler and keeping them is an incredibly good deal. It would be less than a century before their powerbase and skillset are so solid that they no longer have any reason to waste time or attention on anything but ruling well and enjoying themselves, and if they're responsible, they won't let the latter take away from the former. (See Cakelestia.) And it shouldn't be ignored that their idea of ruling well will necessarily involve taking lasting responsibility for the welfare of the nation, so they have every reason to make genuinely good decisions: any bad decisions they make will keep coming back to bite them for centuries to come, or until they fix them. (See Nightmare Moon.)For that matter, even revolutionary America would almost certainly have been willing to elect Washington President-For-Life had he been anything less than strongly opposed, and all the more so if he was immortal.When you combine many-times-proven heroism and selflessness, conspicuous virtue, immortality, and a great deal of personal power, the case for making an alicorn like Twilight a princess is pretty much irresistible.