@KimRegan...no one is leaving the show(I dont know about Leon). They all signed 3yr contracts last summer.
If Abby or Ducky were to be injured it would probably send Gibbs off the deep end. We have yet to find out how Abby &/or Ducky met & started working with Gibbs. What better way to find out than having Gibbs keeping vigil by their hospital bed & thinking back.