Cryptographic backdoors will not work. As a matter of technology, they are deeply incompatible with modern software platforms. And as a matter of policy and law, addressing those incompatibilities would require intolerable regulation of the technology sector. Any attempt to mandate backdoors will merely escalate an arms race, where usable and secure software stays a step ahead of the government.
The easiest way to understand the argument is to walk through a hypothetical. I’m going to use Android; much of the same analysis would apply to iOS or any other mobile platform.