You could say that about most QBs that go that high. Trubisky is a 1 year starter. Goff could have returned for another season last year instead of going pro. Kids today are more likely to go pro once they are eligible.
Luck stayed in school despite also being like Darnold, a RS Sophomore who would be a 2 year starter. But, can you really say that the extra year at Stanford has made him a better pro? How would his career have gone had he turned pro a year earlier and landed in Carolina instead of Cam? Carolina has the Defence over the years.
To me, if I am Darnold, I see who has the top pick. If it's SF or NYG I go pro. If it's Cleveland or the Chargers, I might stay in school. Cleveland is Cleveland, QB graveyard. Chargers, Spanos is a terrible owner. With the rookie pay scale they were still squabbling with Bosa on a contract.
SF, you have Lynch and Shanahan running the show. Offensive minded coach to help me develop. It's in Cali, which is a bonus, as well as a large market. NYG, they have a good D and a big time weapon in OBJ. Worry about the OL and lack of running game. Big market. And MacAdoo will be gone if the Giants are that bad.