The thing that KP could have done that no one else would do was to completely dismantle a bowling line up, or bat a particular bowler out of the game. Psychologically that was more important than how many runs he'd score. Bell (or Key or Shah for that matter) isn't going to do that, even if he makes 3 centuries between now and the last Test. Fortunately for England, the Aussie attack is no better than a moderately good international side at the moment, so an in-form Bell should be able to step up.