Agreed. But, the decision to weight his lack of experience is not accidental or without merit. College starts do hold some weight in correlation to success in the pros. This was a big concern with Sanchez. His QBASE was disastrously low, and we traded up for him because we're dumb. And, no surprise, he was awful. Two things happen with more college starts: 1) Sample size improves for evaluation, and 2) QBs learn and gain more experience in live action against hopefully higher competition than they got in HS.
As for sample sizes, if Darnold has another year in college that looks like his junior year, not his sophomore, you think he's still a top QB? He was smart to come out when he did - pure business decision. I truly believe his camp is very smart, and maximizing his earnings to the best of their ability before he ultimately flames out.