Re: Definitely not for "after civilization ends"
> It was intended to make it easy for signallers in the field
You're right, but either you missed my point, or you conveniently forget those "signalers in the field" were trained (or at very least had the manual).
Did you ever see an Enigma machine? While I know the general principle, there are lots of pieces I have no idea what they do or how to use them, like all the plugs and cables on the front, or those small levers inside the cover. Of course you can assume they are all already configured as required, and you only need to type your message in, but that's preposterous.
I admit I haven't actually tried, but I'm pretty sure that without the manual, nobody will even know how to set it up, much less "reliably encrypt and decrypt messages". So, to get back to the "after civilization ends" part, when and if you find an Enigma machine in the smoldering ruins of some rich guy's mansion, there is little chance you can use it to give the resistance movement an edge against the alien/zombie/whatever invaders.