This isn't the whole picture. There is also the "bottom up" theory of kingship in mediaeval political thought - that the monarch owes their position to the support of the nobles whom they rule. Somewhat like a warband leader. The conflict between clerical endorsement and lay election drove a lot of political action.Historically, monarchies almost always drew their legitimacy from the endorsement of the Church, who spoke for god. Divine Right of Kings and all that.
The biggest thing for me that comes out of that thread is that D&D relies on many non-modern, non-liberal moral tropes.Spinning off a bit from some discussion in the Dragonlance thread.
Many D&D campaigns involve paladin rulers of good kingdoms. The idea that it is possible to have good, admirable government by way of monarchy isn't reconcilable with liberal democratic values. It's the quintessence of reaction, though a very common fantasy trope (Arthur, Aragorn, even some versions of Robin Hood and King Richard).
A lot of fantasy motifs - whether taken from Homeric tales, or martial arts films, or less romantic mediaeval-type stuff - emphasise honour, loyalty, and similar values around personal charisma and personal standing. And consensual submission to violence (eg duelling; or the rather casual use of lethal violence against those who are themselves ready to use it, like bandits and hobgoblins and castle guards). There are also issues of hubris and humility. These sorts of values don't fit very well within a humanist morality.
Maybe with a bit of squinting and clever argument bits and pieces of this can be reframed to fit within a modern value system. But to me it makes more sense to acknowledge that when we play a fantasy RPG, we imaginatively project ourselves into a different, "mythic" or "romantic", moral universe.