If he's still crushing AA at the end of the month, I don't understand the hesitation. The bat might be ready to produce big time at the MLB level. 4 months of everyday PT with 2-3 starts a week at 3B won't be worse for his development than spending the time in a league he's too good for, and it comes with the added benefit of possibly adding wins to the MLB club.
If he gets called up this season, he hits free agency before the 2025 season, at age 26. Are they really going to wait until mid-April 2019? If they do that, he hits free agency before 2026, at at 27. They're going let him annihilate the upper minors for over a year, just to get control of his age 26 season instead of his age-19 season, when he might actually make the difference between playoffs or no playoffs in his age-19 season? I'm sorry, but that's a mickey mouse way to run a team - especially given that this team doesn't have to operate that way. If he's Miguel Cabrera, are they going to let him walk via free agency in his mid 20s?