You've never played in a game with new / first time players who didn't know the rules? I'd argue that they couldn't effectively run the game. (At that point, anyway.)I'm not even a little bit understanding about the difficulty of a DM's job. Like, it's easy. Anyone who can play a role-playing game can run a role-playing game.
Pretending that runnin games is some kind of complex or difficult endeavour that only a few irreplaceable people can do is, uhm, delusion of grandeur, I'd say.
Not throwing rocks, as everyone is entitled to their opinions- I'm genuinely curious: Have you ever game mastered before? Run a campaign from start to finish (say, at least 10th level or so?)
I wouldn't even disagree with your third sentence, except to say that "Anyone who can play a role-playing game can learn to run a role-playing game."
And often, it's not about the difficulty of running the game, but the amount of time and effort invested away from the gaming table in order to make a fun, enjoyable game happen for everyone else... who just shows up to play.