>There are savants who can do amazing feats of mental arithmetic, yet have severe mental disabilities in other areas.

There are also geniuses who can do amazing feats of mental arithmetic and have no severe mental disabilities in any other areas. There are people who have perfect memories (and usually, they live totally normal lives but for the inability to forget a single event). There are people who are far better than average at recognizing faces, pattern recognition, and other tasks that are non-conscious.

The notion that there are "fundamental limits" that are for some reason near the typical human is the cognitive bias of the just-world fallacy. The world is not just. Bad things happen to good people. Good things happen to bad people. If there is a fundamental limit to how smart a mind can get, it's very, very far above the typical human, because there are exceptional humans that are that far away and there's no reason to think a computer couldn't beat them either. Deep Blue beat Kasparov.

>The humans who are in charge of our current society by virtue of having the most wealth, political power, or popularity aren't necessarily the smartest or quickest thinkers, at least not in the way that most AGI researchers seem to be targeting.

I'm not at all familiar with AGI research, but while there are people who start out ahead, there's no reason to think that you can't actually play the game and win it. Winning challenges of acquiring wealth or political power or popularity are related to intelligence, insofar as the concept of intelligence is defined (as it is at least in psychology) as the ability to accomplish general goals.

Walking into a meeting of the Deutsche Arbeiterpartei, joining as the 55th member, and later seizing control of the state requires intelligence. Landing in a dingy yacht with 81 men, bleeding a regime to death, and ruling it until your death requires intelligence. Buying the rights to the Quick and Dirty Operating System, licensing it to IBM, and becoming the de facto standard OS for all consumer computing requires intelligence. Presenting yourself as a folksy Texan when you're a private-school elite from Connecticut and convincing the electorate that you'd have a beer with them requires intelligence. All of these outcomes are goals, and accomplishing them demonstrates an ability to actually put the rubber to the road.

You're confusing the technical term of intelligence, which means something along the lines of "ability to accomplish goals," with the social construct of "intelligence" which means something along the lines of "ability to impress people." Intelligence is not always impressive, and humans have a fantastic ability to write off the accomplishments of others when they want to make the world appear more just. I mean, nobody feels good about the fact that we're one Napoleon away from a totally different world system that may or may not suit our interests. The belief that the world is somehow fundamentally different to the extent that the next Napoleon-class intelligence who isn't content managing a hedge fund and just increasing an int in a bank database can't actually redraw the world map, is just an illusion we tell ourselves to make the world we live in more fair, the stories we live out more meaningful, and the events of our lives have a little bit more importance.

AI could change things very, very drastically. It's right to be afraid.