Re: df is called even if dired-free-space-program is nil ...

[Richard Stallman]

    that's why `df' (or file-system-info) was called
    there, not in files.el.

Actually that's not true.  It was called in *both* places before.  The
practical consequence was that the code in Dired was never executed on
GNU and Unix systems, because the code in insert-directory always ran
first.

    If you want to leave the things as they are now, I think we should add
    some code in ls-lisp.el to make its version of insert-directory do
    similar things.

I agree.  Would you like to do that?

    Is it a good idea to leave a defvaralias in dired.el, for back
    compatibility?

That is reasonable.