A running battle with the Borg cube seems odd given that Borg ships are faster at ftl speeds then the Federation's. However, the Borg's attacks haven't really made all that much sense anyway.
Perhaps the nature of the running battle involves a series of interceptions rather than Fed ships keeping up with the cube. This is just speculation, but...
1) Some ships, including the three rofeta mentioned, engage the Borg in the Typhon sector. Eventually the Borg gets past them and continues on it's way to Earth. The Defiant, and maybe some others, pursue.
2) Some more ships, already a bit closer to Earth, intercept the cube. They don't stop it, but they do buy time for Earth's defences to be organised. Perhaps the pursuing ships catch up, perhaps not. Sooner or later, the Borg fight through and continue on their merry way.
3) Repeat step 2 a few times as.
4) The Borg get to Earth, and are met by the defence fleet. During the battle the Defiant, and maybe others, catch up and participate.
As I say, pure speculation.
As to the OP, I consider it unlikely that the Defiant would be decommissioned. Even with Star Fleet returning to it's exploration and diplomacy agenda, it still has a duty to defend the Federation. Having lost so many ships in the war it seems foolish in the extreme to get rid of any more, even if they are of a type you hope you will never need.