But of course there is. Imagine the following compression scheme:
0-253: output the input byte
254 followed by 0: output 254
254 followed by 1: output 255
255: output 10GB of zeroes
Of course this is an artificial example, but theoretically it's perfectly sound. In fact, I think you could get there with static huffman trees supported by some formats, including gzip.
What you suggest is saving the information somewhere else and putting a number to represent it. That is not compression, that is mapping. By using this logic, one can argue that one bit is enough as well.
