It’s called a Meritocracy. Give people who have the right qualifications and credentials to show what they can do, judge the results, and either keep or move them out. Very simple. Mattis was unquestionably qualified and has done some good things, and failed in some regards. He’s also served in the military for 44 years and as SecDef for two and is 68 years old.
Not surprising at all for these positions to change. Trump is running about the same pace as C level corporate executive placements.
True. I read a study a couple of years ago by the (improbably named) executive placement firm Challenger, Grey & Christmas, saying that the average tenure in an executive position in the U.S. was a little over 2 years. It probably has something to do with accountings being made quarterly. Stock price down for two quarters? You're out.