Nobody could know. But while Baker and Allen were big risers in the predraft process, Rosen slid. Darnold was highly rated from the start. Everybody knew we needed a QB desperately. Bradford, who couldn't even walk properly the last time he played for the Vikings, on a 1 year deal and Mike Glennon of all people. The Cards leaving the draft without a legit future QB would've resulted in empty seats. We were almost forced to take Rosen, and it was so obvious that even Keim couldn't botch the opportunity.
Rosen didn't slid just because of his personality - which was way overblown - but because of the flaws in his game. Surpassing 60% completion just once in college (62.5%) isn't really good, even in below average circumstances. Too many easy missed throws. He continued this bad habit in the NFL. Again in bad circumstances. But circumstances are hardly ever perfect. Easy throws must be completed no matter the circumstances. If you miss easy throws regularly, you won't surpass 60% completion in the NFL. You can get by if you make many "wow" throws to compensate, but one "wow" throw every other game isn't enough when you miss so many easy ones. There wasn't any competition for Rosen at #10 and Keim rightfully pulled the trigger at a favorable price. His arm talent was just too good to pass up at this point. And of course you must have confidence in your decision to grab a young QB.
All this shouldn't stop us though from selecting Murray 1st overall, if he has a higher grade than Rosen.