A huge difference between Mormons and JW's is in counting membership. JW's still count those "publisher's record cards" for their numbers. So their numbers are of active members. But there may be quite a bit of cheating on those numbers. Peak numbers would include counting the same person more than once when they turn in more than one slip for current and past months. Peak numbers include the elderly that they decide must still be counted so someone turns in a slip for them. Peak numbers include parents that never go out in recruiting but turn in a slip for their home study with the kids, and others who never go out but turn in an hour.
Also, a huge percentage of JW numbers would be over 50 years old (many way over that) and there just aren't numbers coming in to replace them as they die. So they are in a similar situation as the Mormons, but may be a decade or two behind before it reaches a critical point of shrinking membership. Yes, that could be the future for the Watchtower.