Replacing the Defiant makes sense, but they replaced the Defiant with another Defiant class ship which they then renamed Defiant and gave the ship the Defiant's exact registry number. I understand why they did all that for budgetary reasons, and at least they changed some of the colours on the bridge, but it was a little underwhelming. It would be like replacing Jadzia with Jadzia's identical twin sister who is also called Jadzia.
Well, I guess that goes back to the question of whether or not the ship should be treated as essentially a character.
I do get the argument that some more emphasis on the new ship being different might have worked better, though.