That's a good argument, but notice what I'm saying is that stealing the bitcoin wallet keys is the only known way to steal bitcoins (I'm not saying bitcoins cannot be stolen, nor that it would be wrong to phrase that bitcoin was stolen as the result of compromised keys). If you're hiding gold under a bed one could steal your door keys, break the door through brute force, set your door on fire, get in through the window, make a hole through the floor under the bed, wait until you get the trash out and sneak in, and so forth. Bitcoin allow you to be concerned with only one thing: keeping your keys secret.