It’s often a good business practice to replace someone when you have a replacement. Trump seems to me to have been following this with many of this appointments and “firings.” They aren’t really so much firings as delayed from the traditional clean sweep of a new administration. He left them in place until a replacement was lined up, with much of the focus on getting the appointments that were attracting opposition confirmed. At least, that’s how I’ve seen it.
Comey, though, is outside of that. Keeping him on for any period of time was not a good idea. I think The Donald knows that.