Trump often picks people for a particular reason, and when that has been accomplished he moves on to the next person with the particular skill mix he needs at the time. We may never know, but I suspect Bolton was just the right guy to convince Korea that this was serious (perhaps China and Iran, as well).
His first Sec. of State was picked for his relationship with Saudi Arabia - when that was secure, he was not longer the right guy for what was next.
Just a thought . . .
As good as any and better than most. I agree.