I would NOT call Bagley up; he's not good enough, right now, to advance you towards a championship as a starting player. He's an average or slightly below average hitting, error-prone 3B. I also think that he might develop into a guy whose bat can cover his glove at SS; if he gets to 85-77-90-77 in the glove ratings, which he might, I could see living with the errors if it made sense with the rest of your roster. it would not be optimal.
I see you've promoted Kennedy already... he's a pretty unremarkable starting LF, and won't get meaningfully better. (Basically, at 2B he makes a ton of errors and has a slightly above average bat for the position; at LF he's a potential Gold Glove but has a well below-average bat; and he'll lead the world in caught stealing if you bat him at the top of the order, which will neutralize the SBs). I might not even arb him twice. Which means that Burnsy's comment is on target-- if you're only using him for 5 seasons you could have waited until your team is really ready to contend and then promoted him. Besides, he's probably a utility player if you're really ready to contend (albeit a very good utility player).