I find this to be a very silly thread.
How can one be certain of ANYTHING? Yes, it's unlikely that CO
would ever get A340-500s/600s but hey.........you never know what might happen in the future. You can't say "I will never see these airlines order these aircraft" because you don't know what the future will exactly be like.
Even if something is unlikely, nothing is ever certain for sure.
People were very surprised when jetBlue ordered EMB-190s.
People were very surprised when easyJet ordered A319s.
Doesn't it seem strange for a low-fare airline that operates a large 737 fleet to order A319s? I thought it was, but hey! It happened! Now easyJet is supposidly looking at getting a couple 747 classics. Pretty strange, huh?
So to answer your question, I don't know what airliners we will never see in airline's fleets. I haven't traveled into the future to find out.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran