In a vacuum, I'd agree that Alcantara deserves a full season look in the OF. However, the current construction of the major league team doesn't really give him a place to play. The OF currently is on lockdown between Happ/PCA/Seiya, the Cubs already have a promising rookie bat getting full time reps at DH in Ballesteros, and Shaw is firmly in the Ben Zobrist supersub role. It's not like Alcantara has any experience at 1B, so he'd basically be relegated to the 5th OF role, which means occasional starts and pinch hitting.
I get that technology has improved and the Trajekt machine can be used to give Alcantara reps, but, to me, that's like saying a rehearsal is just as good as a live performance. At least in AAA, Alcantara is facing live pitching, with many of those pitchers having ML experience. I'd rather he have that than 5-6 plate appearances a week against live pitching in the majors.
As I like to say, Baseball Is Weird and Injuries Happen. Let him (hopefully) dominate AAA, and either some opportunity will come up due to injury in the next month or two, or he'll be exceedingly valuable come the deadline.