I vaguely remember a rumour that Edward Norton was to replace Keifer for season 4. And I think Tony was arguably a strong enough and popular enough character to have replaced him had they offed Jack in maybe the first or second series. But yet further character transformation will be required before they could do that now.
I'd say at this stage Jack is essential to 24. The show has become as much about how much he can endure and how far he will go in the course of saving the day, as it is about the real time format. You couldn't have had The Shield without Vic Mackey or NYPD Blue without Andy and I think Jack is even more indispensable to this show. I know that 24 is notorious for killing off its characters but I think when they kill off Jack, it will have to mean the end of the show too.