"Which, of course, will change nothing. Have they forgotten that all the WW2 communications intercepted by places like Bletchely park were in clear text, easily intercepted by anyone with a radio, because the spies had first encrypted their stuff by hand using Enigma, one-time pads, etc."
Then it was still transmitted in an encrypted form. The thing about encrypted communications is that they're typically not human-comprehensible, which makes them stand out. The only way around that is through steganography (hiding a message within another innocuous message), but even that has its limitations, especially for messages of arbitrary subject and length.
"Even if "they" could somehow force the internet and every device connected to it to use only non-encrypted messages, those who want secrecy will still be able to encipher their stuff before it gets anywhere near the internet, and one-time pads (or their opensource software equivalents) have no back doors."
But has a bandwidth limit and, again, will make it stand out and make it easier to spot and trace. Plus it's tricky to combine the one-time pad with stego and still make it able to get past detection and mangling techniques while still able to produce a message of usable length. Another consideration. If the enemy figures out even one of the messages is suspect and mangles it such that the other side can't decipher it, you've defeated the one-time pad for the duration because now the two ends are out of sync.