These types of questions are hard to answer. There are too many variables: where you get drafted, health, system, etc...I know it is a hot button topic now with the NCAA trying to persuade the NBA to change the "1-and-done" rules.
In Swift's case, I think back to something Doc said recently. He said something like "if you can play in the league, you'll show that, whether you are 18 or 22." I agree with him. Swift clearly couldn't play in the league and I don't think 4 years of college ball would have helped him. In fact, he probably made a lot more money by coming out early. I disagree with people who think that you automatically ruin your lottery chances by going to school - the physical potential that someone like Swift has does not disappear by going to college. But there is something to be said for going to school and playing against better competition and showing your flaws, decreasing your "upside."
I hope the NBA removes the restriction against 17 and 18 year-olds entering the draft. As we have seen with Bradley and Stiemer, the D-league works. It is starting to become a legitimate NBA version of AAA baseball. I would be fine with rules like they have in baseball, actually. I think if you get drafted out of high school in baseball but instead opt for college, you have to stay for 3 years before you re-enter the draft.