...but then most likely, Khan or one of the other Augments would break free, join up with the others to grab a starship that's docked there and then get away back to Earth to conquer it...
If Khan couldn't hang onto control in the 20th century, couldn't beat a handful of basic humans in the 23rd, and couldn't take out a starship in one-on-one combat... what makes one think he would somehow be able to conquer 23rd century Earth?