Re: default-fill-column

[Richard Stallman]

      Indeed, the functionality of these variables
    is not equivalent with `setq-default'.  `setq-default' changes the
    value in all buffers without buffer-local binding, whereas
    (setq default-fill-column 75) only affects the value in newly created
    buffers.

This is a bug; setting default-fill-column was supposed to be
equivalent to using setq-default on fill-column.
The doc string of default-fill-column says so explicitly.

      If one wants to make new
    defaults for these variables apply only to newly created buffers,

We don't want that--that's the bug.

I think the correct solution to implement a new kind of internally magic
variable which stands for the default value of some other variable.
Then default-fill-column can be implemented that way.  It would
be fairly clean and not that hard.