I've never been fully satisfied by this assertion.
Apple has the ability to push whatever code it wants to whatever device it wants. They can make a version of iOS that bypasses encryption and restrict it to only run on devices identified by a warrant. They could post it on GitHub and it wouldn't matter because iPhones would refuse to run the code if it had been modified by somebody other than Apple to run on any device. You would need Apple's internal code signing keys to actually do any damage.
Currently, the security of your iPhone is dependent on Apple's internal security. If that gets compromised then your phone can be compromised. In a world where law enforcement can get a warrant to force Apple to unlock a specific iPhone, the security of your phone is still dependent on Apple's internal security. Nothing changes for anyone not targeted by a warrant.
You don't need to give law enforcement a universal backdoor key to enable them to execute warrants on devices. What am I missing?