Key sizes are generally chosen so that brute force is infeasable even with enormous speed advancements. You cannot increment a counter to 2^256. There isn't enough energy in our solar system. So you cannot brute force 256 bit symmetric key encryption using traditional computers. Not at any speed.
yeah; to get a sense of how big 2^256 is: assuming you can increment a counter at 1PHz rate, it would take 91 million years to iterate over 2^256 values.
