I sent out both an ugly coin and a gold "pirate dubloon" coin. The gold coin remained in play for a little longer. I sent out coins defaced (my initials scratched into them, holes drilled), I tried attaching extra info, sent coins by themselves (no pouch or anything, just the coin), and sent some in their flip sleeve and in a ziplock and all. I can't detect any difference in planning vs. loss. Mine did OK from 2009-2010, then began to evaporate, and now the majority are lost.
Since these are eminently "Trackable", having tracking numbers and their own pages, they could be picked up by an honest person and returned to the game at any time. Even years from now. I'm not waiting around for that to happen, but it can happen. I also expect any coin that resurfaces to be lost again pronto. So it's never ending.
That's happened to me once before, but I think you have to admit for most geocoins if they go missing they go missing.