I think it's pretty simple, and that's why I called this happening exactly as it did months ago.
#1: This was a terrible draft for fast linebackers. Seattle could have had Arthur Brown at #56 and still traded down, and Brown was clearly the best fast linebacker of a bad bunch.
#2: Seattle already plays a lot of nickle defense and that rate will increase next season thanks to the addition of Winfield. WLB might only see 20 snaps a game.
#3: Seattle has good depth at LB. Smith, Morgan, and Bradford are better players than most casual fans realize because those LBs don't get the playing time to amass sexy counting stats.
Seattle's in good shape. The next year that has a good draft for fast linebackers, look for Seattle to make an investment. In the short term, their LB situation is more than fine.