One problem Bagley faces (which was a possible problem picking him) is the glut of front court players we have.
Even discounting the vets, we have Willie, Skal, and Giles. While Willie and Skal are not everyone's favorites, you have to see where they take you, or what they can get you. A team, particularly one like ours relying so much on kids, can't afford to give up on them.
Suns can (and should) design their plan keeping Ayton's skills in mind. He will know where he will play, how many minutes he'll get, what his role will be, etc. In our case, we don't know if Bagley will start/come off the bench, play 4 or 5, low/high post, other players/bigs he shall share the floor with, number of minutes he'll see, etc. It's not a recipe for success.
Despite this, picking him was the correct choice if the FO was convinced he's the better long term player. Players don't become stars/super stars based on talent alone though. They need the system and structure to maximize their talent. Drafting him is only a small part of the fix. Vlade needs to be active to clean up the roster. If Willie/Skal show good improvement, it might be easier to package them with many of our other assets (space/expirings/huge number of second rounders/some additional kid) to get something of value back.