If they were to stop the book studies in the home and move them all to the hall I think the attendance may remain about the same or drop off, unless they added some other meeting to try and attract the people. The problem with moving book studies to the hall, is many KH's are already congested enough, and this would seem to add further burden to the schedule. Also home book studies are located at different areas throughout the territory, on different days and at times that are supposed to be convenient for those in the group. By making everyone attend a book study all at once especially if it was midweek, the attendance likely would not be too great from those who work 2nd shift.
Also the TMS & SM are probably the second set of least attended meetings to the bookstudy so I don't see how it would help attendance.
Now maybe they are planning on having one congregation bookstudy and adding a meeting for youth after that, but I don't see how that would attract the middle age and older ones.