Ras wanted to destroy Gotham BECAUSE IT WAS UTTERLY CORRUPT(and overrun by crime). By the time Bane came around, that was no longer the case. The city was largely "fixed" and was at peace
You're conflating two issues here. Endemic corruption and the level of violent crime are not the same thing. In fact, Ra's's little speech wasn't primarily about crime. There is some evidence in the film that Bane's group sees the inherent power structure of the city as corrupt even in so-called "peacetime".