Great decisive move by POTUS. As much as we respect Mattis, once he decided to leave then he should leave as soon as possible. SecDef should not be a lame duck that does not have his heart into it.
God Bless Mattis for his service to the country. He is a patriot no doubt. But he knows that he served at the pleasure of the president and is his heart was not into it, then he should do the right thing and leave.
Enjoy your well earned retirement warrior monk.