You guys keep using the successful players with very long careers as examples of "not the worst thing". That's an extremely poor way to look at the downsides, guys who last 15 years and are paid like all-stars are fine no matter when they come out. The difference is when things go wrong with your career.
Where would Greg Oden be if he'd gone back for another year at Ohio State? How much money would Gerald Green or Kwame Brown have lost out on had they gone to college for a few years?
Do you think only negative things can happen going to college? Is there's no possibility for positive development?
While Greg Oden probably has knee injuries either way (although you can argue he won't), a guy like Shaun Livingston who has a freak knee injury, if he's in a different place, maybe he's at $80m+ in career earnings instead of $25m+.
You can definitely argue Kwame Brown or Gerald Green are more successful with a little more discipline and development. One year in college, maybe Green doesn't spend 2 in Europe. Maybe Kwame Brown becomes at least a solid starter, instead of a scrub.
It's a hypothetical argument for sure, but I don't think you can definitely say college only makes things worse.
While everyone points to extremes, we should be pointing to a guy like Marvin Williams. He went to college for one year, when he was likely a top 5 pick if he went pro. Despite going to a great D1 program, his career has sucked. It probably would have sucked just as much had he gone pro straight out of high school. So he'd be at maybe $50m right now instead of $55m in career earnings. Do you think he's sitting around saying that 1 year of college was a mistake? Probably didn't make a difference in his career.
Another year in college likely doesn't make a difference, though it could have positive or negative results. Impossible to know for sure, but fun to discuss.