What DoD needs more urgently than anything else is change its focus from social justice and social engineering to warfighting, and that will require lopping off the heads of a lot of social justice warriors infecting DoD, eliminating LGBTQ sensitivity training, etc. Also, reversing the mandate to place women in combat units and have gender-neutral physical testing.
All that stuff needs to go, and it needs a real leader to make that happen.
Mattis is absolutely perfect for what currently ails DoD.