You are forgetting about the people who build hardware just to crack this. No, not really. for each bit of length you add to the key, you double either
the (average) time or the amount of hardware needed to crack it. as the pool of hardware gets larger, the storage, communications and power requirements grow in proportion - rapidly leaving only the time component to increase. assuming you could build a machine 256 times the parallel processing power of the des cracker, that still only shaves 8 bits off the keylength - and of course the 56bit cracking machine was only capable of testing 56 bit keys - so a larger gate array design would be needed further increasing the number of chips or power of chips needed per parellel testing unit. I am sure if the NSA had built a city the size of new york as one big computer, with power stations and staff, just so they can crack *a* key within the lifetime of its user, someone would have noticed...