The reason for keeping Dermott would be so that the Leafs aren't completely ####ed if Rielly walked. He's a passable #4 defenceman, and between he & Sandin, would have likely had to pick up a good chunk of the slack.
With Rielly locked in, Sandin & Liljegren having more upside, not really / at all surprising.
As another poster mentioned, they really should be looking for a young centre with some upside. Spezza wont be around forever, keeping Kerfoot beyond this year could be a challenge, somebody's bound to get hurt, and Kampf is a pure shutdown guy.
After losing both Brooks and Amadio, they're also quite shallow in terms of centres they can call up.