Here's another legal hypothetical ...
suppose a company devises a circuit which can't be read without the correct key being loaded into a register somewhere. Loading the wrong key causes the device to short and destroy the contents.
Police seize device. Ask for key. Suspect accidentally (!) gets a digit wrong. Police try key, and device bricks itself, and is completely unreadable.
Since the device no longer contains encrypted data (in fact it contains no data) what's the score under RIPA ?
As a rule, if changes in technology can cause a law to become invalid, then the law was probably a bad one to start with.