After my first time I said the same thing, (although my biggest complaint is still that WA houses the in-laws) but I'm a bit different now. In previous years I've never bothered to work out economy and costs etc but this trip I really did a full expense list and the difference between flying and driving for me is now a no brainer.
Of course the time frame can be an issue (but again remember I visit in laws so there is little rush) but this year fuel and accommodation costs worked out about $170 more than the cost of flying, travelling to the airport, leaving the car in long term parking and getting from the airport in Perth to the burbs. But driving also means I can go where I want when I want and don't have to go looking for a car seat for the yougen as well as not having to fit in with other people if I don't want to.
I do the trip casually and are off the road for dinner each night and don't start before the sun gets up and I can still make it an easy 5 day trip. I see no need to push it too hard because pushing it harder just increases costs and time spent with the in-laws but I really can't see myself flying across there again at this stage