While I totally agree that you should keep on to Karlsson, and I also believe that he is the best offensive defenceman in the league by a large margin, I do think the point can be overstated.
Remember, a couple of years back everyone thought Mike Green was virtually untradeable. Now he's pretty far down on most people's lists.
I'd hang on to Karlsson. But don't expect him to do what he did last year every year. Defenceman are un-effing-predictable.
10-team Keeper. Everyone keeps 12 + 1 G + 3 "young keepers" (players with less than 164 NHL games played), and 1G. Points for G (2), A (2), PIM (.2), Hit (.2), Blk (.2) SHP(1), W(4), SO(8) Must dress 4C, 4LW, 4RW, 6D, 2G
C - Crosby, Backstrom, Kuznetsov, Eichel, Johansen
LW - E. Kane, Drouin, Schenn, Miller, Foligno, Zucker
RW - Wheeler, Stone, Lindholm, Oshie, Tolvanen
D - Karlsson, Carlson, Dumba, Nurse, Ceci, Pionk, Larsson, Montour
G - Murray, Quick, Koskinen, Blackwood, Georgiev
Minors - Turcotte, Bouchard, Kaprizov