I thought "mediocre" was the middle one third, rather than the middle two thirds? ;)
Not sure if you're kidding, but GP is referring to area of the one-sigma region around mean for a normal distribution (see: bell curve) in relation to the total area (68ish percent of total), which is usually where the "normal" people live. It seems the popular understanding in soft sciences is that distribution of talent/ability/other human metric seems to follow a normal distribution.
Well if the trend is right, and continues, it means future generations will have to have education and a society that by default works in a critical thinking - always learning - always thinking kind of mode. This could be huge for humanity. I think it's mostly culture and societal norms that dictate the normal modes of thinking, and that most people can do just fine if they are brought up in the right environment. The other option is some kind of transformation or crash, or that technology fails somehow and crashes. Or? Well it could be a very bright future if the conditions are right and the planet allows it