What would have been preferable to replacing the Defiant with an identical copy at the last minute? I quite like the idea of Sisko and crew getting the Venture, or some other Galaxy-class ship, as a fleet command vessel for the final push.
In order to emphasise the loss of the Defiant, I would have had Sisko and Worf aboard the Rotarran rather than just getting a brand-new ship out of nowhere.