You can't assume that Castro would have turned out better handled differently but you certainly can't assume that he wouldn't have either not unless you want to flat out dismiss the concept of player development.
That being said, The Rockies are the ones who really erred here not the Jays. After his failed and rushed audition, there was no reason he couldn't have been put back on the development track he was on previously. I never understood why the Rockies plopped him in their MLB bullpen repeating a move that had just backfired on the Jays.