I cant say it was inevitable because nothing is a total certainty in war, but it was unlikely, Sherman had practically all the cards in his favor during that campaign. Had the Cassville affair, or Peachtree Creek or another one or two of Hood's battles been better executed by the Confederates then the fall would have been delayed at least, but Sherman had to be defeated to save Atlanta and he was far too sensible and cautious a general to give favorable odds for anyone to defeat him in those circumstances.