Had this discussion with Rob Cortaino recently - "Sound Design" and "Film/TV Sound Recording" are like night and day in comparison. Film/TV is a specialist production craft whereas "sound design" is simply a catch all for anything to do with sound.
It's not going to draw the same audience at all. By specialising this craft we are going to draw in experts from the field of film/tv production sound that would not generally be attracted to the sort of material that is being covered in "sound design".
It will build audience, not siphon it. Certainly there will be some questions that have been asked in "sound design" that would be appropriate for the new forum, but that's to be expected as there isn't an alternative right now.