I was not referring to low chance drops like Onyxia, but 100% drops for certain achievements... Alone in the Darkness, or Sarth 3D. I don't think the two are the same, and have little to no doubt that the low chance drops will stay in the game.
I hope they keep the mounts in the game, to be honest. At the very least, as a reason to revisit old content now and again beyond getting the associated achievements.