Another problem with your argument is that, in the aggregate, people can be more rational than they are individually. An example of this is the so-called Delphi Effect in which the average opinion of experts is a better predictor than any single expert's opinion.
Now, those are pretty theoretical arguments and have issues, some easy to address and others hard. However, I claim that any social institution or theme that is as widely practiced in space and time as government has a net social benefit. The question is how to tune it to maximize that benefit.
That statement is true as far as psychology; people are incapable of thinking in a purely rational manner. They're too recently descended from the trees; sometimes they still can't shake their need to bare their teeth and squabble over bananas and females.
As pertains to an argument, it either is or is not rational. There's no such thing as a "slightly rational" argument.
Suppose all the inhabitants of a country were, as you suggest, under the authority of a non-rational being.
In other words, pick any country in the world, and suppose that it's just exactly as it is today. Check.
That is, this one being made every decision for everyone.
Who said "every" decision? No dictator in the history of the universe did that, nor ever would.
If this being were more rational than everyone else, every inhabitant would be better off being ruled than if they made their own decisions.
Fair argument, but it rests on a false premise. Namely, that this "more rational" person's idea of "better" really is "better" for everyone. For example, any rational person would hate rap; therefore a "more rational" ruler would of course ban the nasty garbage. Unfortunately, that makes his rap-loving subjects miserable. In what sense are they therefore "better off"?
Wellness itself is not a rational concept. Vanilla makes me happy; chocolate, you. It is inherently impossible for any human to make better decisions for another than he can make for himself, precisely because each person's definition of "better off" is different, and nobody's is specially blessed by heaven.
Another problem with your argument is that, in the aggregate, people can be more rational than they are individually.
That doesn't make them better at achieving a non-rational goal, namely happiness. If everyone in the country (except you) carefully voted on every aspect of your life, we could not improve your happiness better than you could by being left alone.
However, I claim that any social institution or theme that is as widely practiced in space and time as government has a net social benefit.
Your task, should you choose to accept it, is to prove that.