Generally, I don't agree with arguments of the form "If we ban X, only the bad guys will have X". (For example, if X is "guns", then total general unavailability of them, would eventually drive manufacturers out of business - and sooner or later all guns (and ammunition) would rust into non-existence and the bad guys wouldn't have them.)
But crypto is different. It's math. Since the math is already "out there", it only takes someone with roughly college-level software skills to turn that math into code.
So it is truly the case that making crypto illegal will simply result in no-back-door-crypto software appearing on the dark net for low-$$$ and the bad guys will have it.
At that point, the lack of crypto in the open market is irrelevant. The NSA (et al) will still need to decrypt the bad guy's messages the hard way.
And if we're honest about it - the real bad guys can probably figure out how to use a one-time pad correctly - which is easy enough to do and unbreakable.
Another issue here is how the law would get rid of all of the crypto that's already out there - do they seriously expect everyone to load new software for every device that uses cryptography in any form? The cost of that would be staggering!