Network hash must grow at least at the rate of Moore's Law. This is also known as Sweft's Law. If Bitcoin fails to satisfy this condition, then it will be prone to attack.
Why wouldn't hashrate grow according to Moore's law. If hardware performance increases in hashrate per $, everything else being equal, why wouldn't the bitcoin network hashrate increase accordingly and Sweft's Law be satisfied?
There will always be new miners entering the game with new up-to-date hardware (or existing miners upgrading) and old rigs will drop out, keeping bitcoin hashing equipment up-to-speed with moore's law.