In other words:
This relies on many tenuous assumptions being done by an elaborate chessmaster.
Fenris is neither a chessmaster, nor someone with a long attention span, nor someone willing to so much as sit around for a few centuries guiding incredibly fast reproducing entities to conquer the lands.
I doubt he would have a master plan here, he just made an impulsive move early in the world's creation and then abandoned it. A few millennia later, The Dark One showed up.