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.
Of course there is. We are bombarded with them daily. Haven't you been reading the DP/ports threads? I'd probably go further and say that there's never been and never will be a significant argument that's purely rational. Eventually the choice must be made on a value basis and values aren't rational.
I'll ignore the nits you picked and get to a real issue, is government socially valuable? I think the evidence for it is compelling but of course one can't know for sure. It may be that, perhaps compelled by an evolved tendency toward hierarchy, people throughout recorded history have chosen to form governments even though it is bad for them but I doubt that. People aren't that stupid (well except maybe liberals). One of the greatest social inventions, rule of law, requires government. People cannot effectively organize for their common defense against an organized enemy without government. Simply having to debate policy rather than acting without thinking is valuable.
But of course government is not uniformly positive and, as I said, our goal should be to shape the principles of government to increase its social benefit.
I just realized I failed to make an important point. The reason for my first reply was to cast doubt on your argument (although you neatly tried to cast the burden on me). It seems to me that, in the end, the question must be answered empirically (as every interesting question must). We have a tremendous amount of experience with government. It is up to you and people of like mind to *show* the rest of us that an ungoverned society of significant size can truly function more effectively than a governed one. I for one would welcome the experiment.