But is it an accident ?
A world with no humans and just machines could be seen as the evolutionary goal of a self aware machine. Much as a world with just humans and no nature seems to be ours.
Who knows ? Maybe it will turn out we failed to grasp the implications of evolution and needed AI to explain it. Practically ?
If you have devoured as much science and science fiction as I have (as well as various religious ramblings) then you can appreciate the Bards:
"There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."