It is highly intriguing how Puella Magi Madoka Magica, despite being very much physically incorrect, is very ecologically correct. What I mean by this, is the fact that it takes into consideration how witches filled some sort of an ecological niche. When the witches are erased by Madoka, the niche does not disappear without a reason, but it is inhabitated by different organisms, adapted to the same environment, and displaying similar dietary preferences.
This serves as a good worldbuilding tool too. Assuming that the first magical girl became the first witch, we ought to ask ourselves what was the reason behind her decision to become a magical girl. While she could have agreed just for her wish to be granted, I don't think that she would trust Kyubey enough to believe there is nothing she would pay for that.
And that is where the wraiths come in handy, because the situation from before the witches appeared was in some way the same as when they had disappeared. That means Kyubey could just use the same tactic as during the events we know, but with wraiths instead of witches.
