As a mere point of interest, that is logically equivalent to:

  When not only the criminals have X, then X is not illegal.

When X is anything, criminals will likely try using X for bad things. :D

when murder is illegal, only the criminals will murder

"When nuclear bombs are illegal, only the criminals will have nuclear bombs."

I think your grand unifying theory needs more nuance.

I hope we don't depend on the unavailability of nuclear bombs to the bad guys on them being "illegal".

No, we depend on the enforcement of that illegality, which is usually assumed when people discuss banning something.

(EDIT: Previously said "making something illegal", which I realised was ambiguous.)

What, do you plan to make talking about crypto illegal too?

I was replying to the general sentiment, not the specific.

Obviously banning encryption is ridiculous, but the "Only criminals will have X" argument is just silly rhetoric.

EDIT: I just noticed the ambiguity you're more likely responding to in my original post. I'll reword it.

I think your nit-picking needs more thought, nuclear bombs are illegal for an individual to possess, and if someone had managed to obtain a bomb from say the former USSR, they would most definitely be considered a criminal.