I disagree about Maguire. I think you have hit upon the reason for his not being as developed as other players of the same age when you mention the number of games he has played. You need games at a good level to aid your development. Over the last few seasons under Robinson, he had McHugh, Gorrin and Donnelly ahead of him for his best position, players Robinson liked. I'd hazard a guess he has played as much under Alexander in one year (and he's been injured a spell) than under Robinson and he has improved. Since his injury against Hearts, we suddenly became a lot easier to play through. For me, he seems to know his limits and is less likely to abandon his defensive midfield position.
On a general point about young players (and this isn't aimed at anyone in particular, just a general thought), there is a tendency with young players coming through that if they're not the star of the team or likely to bring in a transfer fee to brand them "not good enough". There's a place for squad players and to use Maguire as an example, if you don't think he's good enough to be a regular starter, is it not better having him on the bench than paying a signing on fee for a Lee Lucas, Craig Clay or another random player with no real affection for the club...