@uniaka:
While I could agree most of the time, I just can see the first 1 billion bounty introduced and defeated on the same island. For years Ace's bounty was the first, with confimation that above 400 it only goes up very slow, Luffy got the 500 one a few chapters ago, then jack shows up with 1 billion.
With Jack presumed dead, we have an excuse to have Jack defeated without giving Luffy another raise.
Which is again another reason why having him being defeated on Zou would be benefitial, as no outsiders would be aware that he still lives.