I had been thinking the same thing for a while but chose to ignore it because i didnt want to believe that Luffy's haki was so common. Though having just read your post i rethought about it.
Think about it like this - Take any world class sport, tennis, say. Now think about how many people have the potential to be the "best in the world". I'd say around one in a million, if you take an average of the whole worlds population, has the "potential" talent. So of the 7 billion population, that would make around 7000 potential candidates. But now we have to think; How many of them actually want to be tennis players? How many of them have the resources available? Also, just because they have the potential to do something, its just a potential at the end of the day and without hard work and training it will just go to waste.
If there are others like Luffy (and there are) only a handful will actually be able to use their haki properly. Even Luffy doesn't know how to, yet. So don't worry about it too much. As another comparison, say Luffy met some other dude who happened to have the exact same DF power, Luffy would still destroy him. Same is applicable to haki, though currently Luffy is pretty useless with it - Boa > Luffy :P haha
As for prediction, I agree with most that Shiryuu will end up with BB one way or another. But I think BB's main reason for being there has got to do with the bounty poster Don Flamingo was looking at while they were all eating together. You can see BB's eyes overlooking the poster. Assuming it's Luffys, with some mega bounty, post hitting the tenryubito, then BB is there for Luffy - rumour spread he was in ID.
Note: For those thinking Ace is already at Marinefold because they were at a set of gates - there are 3 gates, one at each vertex of the triangle. They were just about to enter the current which will take them to headquarters.
We shall see soon enough
). So, imo, Sanji's departure is equally dramatic for both the anime and manga.
