"Shell shock" becomes a euphemism when you're talking about people traumatized by something other than artillery. PTSD is more general, and therefore more precise in the general case.
The terms "in shock" and "shocked" are not equivalent. The former means traumatized, while the latter means someone is extremely surprised.
You got an atomic bomb dropped on you, experienced a literal apocalyptic sun consuming everything you loved first hand and all you (impersonal) can come up with is "stress"?
If you want to stay coherent then we should have made up a term specifically to tackle the world-ending experiences these people lived through.