There are tonnes of other approaches to machine learning that don't involve backprop! There are also other approaches to neural netwroks that don't use backprop

There are tonnes of other approaches to machine learning that don't involve backprop! There are also other approaches to neural netwroks that don't use backprop (have a look at Numenta's stuff for example). I suggest you watch Pat Winston's brilliant MIT AI lectures to see how huge the range of techniques is.