Predicting who will be college and NBA stars is difficult when you are evaluating 17-18 year old kids, whether they are at the top of their draft class or well down the line. Your point is a good reminder that recruiting rankings only tell you about the raw materials you are starting with — they don't predict or guarantee college or NBA success.
But I'd rather start with kids at the top. More of them will make it than those lower on the list.