D&D is a generic fantasy game. The rules can be used to play any type of game that you want to play. If the system wasn't able to be used beyond hack-n-slash generic fantasy, there would be no d20 market.
Besides, d20 Modern dosn't really cover our world any more than D&D covers our past. For d20 Modern to cover our earth, you would have to ditch all of the campaign models as presented, as well as all of the FX.
It would be like dropping Magic, psionics, and Demi-humans from D&D. Once you do that, you can do an "earth's past" game pretty easily.
D20 future was needed because there are lots of things in sci-fi fiction that cannot be emulated using the rules in d20Modern. They had to design systems for cyberwear, mutations, space travel, ect. There are not to many things in the time era covered in d20 past that can not be done using d20 modern. As I said earlier, a 96 page book sounds about right.