Gotham's far from being fixed when Bruce leaves it.
I mean, think about it. All the criminals Bane freed are still running about, and arguably could avoid being reimprisoned by arguing the Dent Act unjustly put them in prison in the first place. Will the people really still trust the cops after Gordon had told them all a lie for eight years? One day of heroic bravery by the police in fighting Bane's army won't make up for the near decade's worth of lies they had been told. There hasn't been any real reform in Gotham, just temporary bandages, like with Bruce's parent's death, Harvey's cover-up, and now Batman's supposed demise. Hell, criminals might be even more active, since Batman was a big obstacle in keeping them in check. The cops could only do it effectively because of the Dent Act, which, as I mentioned, is completely built around the lie.
In a way, Bane and Ras are right: Gotham just keeps covering up it's gaping wounds with Band-aids, and either needs someone to reform it from within completely, or put it down.