What is the advantage of super complex automatic transmissions in passenger cars? Such as 7,8,9 and even 10 speed transmissions.

Depends on where you live. Most people live in cities, where stop and go travel is most common. If, for example, you spend 1/3 of the distance between stop lights accelerating, and 2/3 cruising -- then getting up to speed is still going to be your largest point of consumption. Thus having more gears will reduce that consumption. But even if you lived in a rural environment, having 3 different overdrive options is still more efficient than just one overdrive. Things like elevation changes will lead to changes in engine output requirements, so instead of dropping back down to 1:1 to climb (ex: this is my 7th gear in my 10 speed) it's still more efficient to have multiple overdrives when cruising. My 10sp often only drops to 9th, maybe 8th gear on a climb if I'm not pulling or hauling something. And even when pulling or hauling on long distance, having more gears to choose from for a given ratio depending on the engines need will lead to better more optimal performance and better mpg. Unless you live and only travel on completely flat surfaces, in areas devoid of external influences like wind, hills, traffic, stop signs, or traffic lights etc. Then maybe you only need one overdrive to make a 6 shoes as good at getting optional mpg as a 10 speed. But that's not how things work in the real world for the overwhelming majority of vehicle owners. I just can't possibly think of a real world situation where a 6 speed with a a singular (and equal) top overdrive ratio to a 10 speed can be anywhere near as efficient.