I am going to throw something crazy out there, even though I do not necessarily believe it. Maybe the Gold Roger that was captured may not have been the real one, and maybe the real one hid underground as Monkey D Dragon. I mean, who would know more about the world than its truth than him? I would not put it past the marines to pull a fast one on people just to prove their point (Gold Roger died). I know it is a bad theory, since many people saw it, and like I said, I do not necessarily think this is the case. Just food for thought (after considering that whole making Ace's execution super important).