Re: Entering filenames with spaces

[Richard M. Stallman]

    I think you are right.  This is not strictly filename specific.
    Spaces and question marks can occur in non-file completion strings
    including buffer names, Info index items, function names.

Yes, they can.  But entering these spaces is not a problem; just type
SPC.  As for question marks, entering them with C-q is not hard
either, on the rare occasions when it is needed.

There is no reason to consider such an incompatible change.
Let's not discuss that further.