If you don't know (or trust) the encoding, you have bytes, on which you can do whatever heuristics you wish, then decode into a string. It's much safer than changing the encoding on an existing string.
How in the world is it any different? #force_encoding is not meant to mutate the data. The entire point of it's existence is to be able to correct a wrongly set encoding.
If you don't know the correct encoding of a sequence of bytes, then you don't yet have a string, just a sequence of bytes. You can decode the bytes into a string, or encode a string using a particular encoding into bytes.
