Cricket Australia has taken a bold and presumably the wise decision to lessen the workload of their top 3 seam bowlers, Mitchell Starc, Pat Cummins, and Josh Hazlewood. Team management has decided not to play all the 3 seamers together in the same Test match.
One of them or probably two will sit out making way for the other guys like James Pattinson and Peter Siddle during the Ashes 2019. This decision will help them prolong their careers as they have already played a lot of hectic cricket together.
Mitchell Starc had a phenomenal ICC Cricket World Cup 2019 and he might need some rest before making a transition from the ODI format to the longest format of the game. Josh Hazlewood has recently recovered from an injury and has not played a significant amount of cricket in the recent past.
The lesser the workload, the lesser will be the injuries and hence more efficiency from the big 3. This sagacious decision highlights that Cricket Australia is planning on a long-term basis and they don't want the team to be heavily reliant on Starc, Hazlewood, and Cummins.