I think a good thought experiment might be to consider what might happen if they completely stopped making converts. How long will they survive?
Unfortunately, the answer might be: quite awhile. There are an awful lot of people who came in pre-1975 still out there. Many of these converts had good jobs already and are now enjoying a good retirement, my parents among them. I expect to see a greater push towards including the Watchtower in estate planning.
This could keep them going for quite a few more years.