I wasn't comparing the players. I was saying that Brohm stayed in school and it adversely affected his draft stock.
I think that Darnold may have already seen some of that (though top-5 may still be salvageable with good workouts and interviews), and if he goes back to school but doesn't take his game up a level, he could end up suffering the same fate as Brohm on draft weekend 2019.
Jackson, IMO, is a different case. He's facing the question of whether or not he can play as a pocket QB. If he returns and doesn't show an improvement, it won't hurt his draft stock, because that won't be a new development. He'll still be considered a great athlete that can make a subset of NFL throws, but hasn't shown an ability to win games as a dropback passer. On the other hand, if he returns and does show an improved ability in the 2018 college season, then he's probably going to be a top-5 pick.
You're right about the gunslinger mentality though.