I can't imagine them stopping the door-to-door work, unless they find an adequate replacement. If they cannot recruit people at a sufficient pace, growth will slow and incoming money will slow and that could start them on the long road towards irrelevance.
Money seems to be the real issue, since they have the large recording site and other offices both in the USA and around the world. They also are dealing with CSA cases and those are a drain on resources even before any potential settlements or verdicts. I suppose they could scale things back and keep selling real estate in order to keep their heads above water. But it's kind of a snowball effect-- spending less money means less output and lower quality, which can make it harder to get more money, and so on.
But I get the impression that there are many small Christian denominations that broke off from the Adventists and JWs and are much smaller and lesser known, and they have survived for a very long time. The WTS could conceivable shrink and become small but still exist and still exert tight control over the membership. They would not be very relevant anymore, but they'd still be around.