The great military purge is a myth that has taken on a life of its own. But, on your second point, you are completely right. The current senior leadership have passed through a filter that measures their willingness to become complete sycophants. Forget the guys with zipper or integrity problems. Not a single senior leader in uniform has resigned on matters of principle, they have had plenty of opportunities to do so.
If you are not a sycophant willing to proscribe to Obama’s perverted policies, then you are figuratively cut out of the herd and cut off at the knees. Call it being villified, ignored and let go. It’s not a purge in the Stalinesque sense of the word, but the effect is the same.