Rask has only been called up for brief stints this season when Fernandez was hurt. Neither time was more than a couple of games. Chiarelli knows that it makes no sense to bring up your future goalie to play in maybe 4 or 5 games down the stretch when he could play every day in pressure games in a playoff race in the AHL. I also said he'd be the backup for 1 year, not 2. I said in year 2, he'd be the starter (this assumes he continues to develop).
As for letting Fernandez walk - what do you expect to get for a goalie that everyone knows will be a free agent in a few months and turns 35 in the off-season? As for his stats - yes, they are great, but he's only played 20 games. This is his 13th year in the league, and he's only played in more than 44 games in a season ONCE. He's a backup now, not a starter (and you could argue that he's never been a true starter). Teams won't exactly be busting down the door to acquire him. More likely, Chiarelli trades his rights on draft day for a mid-round pick.