You are missing the point. Australia would never have won those WCs if they `always' treated it as less important than the Ashes. The fact is that currently they cannot look beyond the next series because they must get back to winning immediately in order to go into the WC with any sense of confidence. The last thing that they want is to lose to another Asian side at home. Them playing the best possible XI for this test series is a no-brainer and has nothing to do with them attaching more importance to the Ashes.
Even you are missing a context here
Australia won WCs because other teams were not capable of winning it. They lacked quality. In last 4 out of 5 WCs they played Pakistan in 1999 WC Final and we do know how Pakistan team performs, they are inconsistent !!!! In 2003 WC they faced India who were never a good team in Finals in pre-Dhoni era. In 2007 they played Sri Lanka who always succumbs to pressure in must-win games eg- 2003 WC SF, 2009 WT20 Final, 2010 Asia Cup Final, 2011 WC Final, 2012 WT20 Final, 2013 CT SF and many more crucial matches were lost due to defensive Cricket so I am not surprised that they won. Also Australia played 2015 WC at Home while the only match they played away from Home was
Lost in a close contest. Australia never wanted WCs they wanted dominance especially over England who once ruled their nation. Again I say Ashes means more to Australia than WCs. They can afford to lose WCs but losing Ashes is always something that doesn't please them