What will be the benefit?
- CHAR_BIT cannot go away; reams of code references it.
- You still need the constant 8. It's better if it has a name.
- Neither the C nor C++ standard will be simplified if CHAR_BIT is declared to be 8. Only a few passages will change. Just, certain possible implementations will be rendered nonconforming.
- There are specialized platforms with C compilers, such as DSP chips, that are not byte addressable machines. They are in current use; they are not museum pieces.