737s and 767s in the west now operating on routes developed by the 717.
The 717 cannot replace the 737-800, it is only suited for the thinner routes. Maybe the Canberra and Hobart routes can no longer support a 737 but are too much for a Dash. The 737-400s are tired and worn and unpleasant from a passenger perspective and the 800s are too big for some of these routes at certain times. The 717 fills the gap perfectly.
Not all decisions are made solely to screw mainline crew.