This is a well known problem I described back in 2011 and extensively wrote about it.  I created a law for all proof of work crypto currencies that is known as Sweft's law.

Sweft's Law:  The network hashrate of a proof of work cryptocurrency must rise at a rate at least equal to that of Moore's Law to provide minimal network security.

The thread is here of anyone wants to read it.


https://bitcointalk.org/index.php?topic=12109