Regarding his performance in A-ball, he is young for that level and it’s not unheard of for players to struggle when they are first promoted. It is possible that he’ll look better by this time next year.
That being said, most of the hype I’ve seen about him has been about his bat, and even then I’ve seen his scouting reports point out the lack of loft in his swing and that he may need some changes in his swing mechanics in order to fully tap into that raw power. Also, at least based on what I’ve seen on Baseball America, it seems that although his plus-plus arm may be helpful in RF, his speed and defense is generally seen as below average. While it is possible that his defense improves somewhat over time, he’s likely not getting any faster, especially as he bulks up with physical maturity. Perhaps that’s what factored into why Arnold thought he was expendable.