When those elderly, die-hard ones are gone, they will be left with a younger generation that's just not as devoted with their time and money, more materialistic, more prone to send their kids to college, and who can't live without the internet. These people may attend meetings, but not make all of them, go out in service, but only occasionally, donate, but not generously... in other words, a new group of watered down Witlesses emerges. They won't be as gung-ho as the old ones were.
I predict more down-sizing ahead!