I think it is flawed reasoning to assume a constant (or linear) rate of decrease. I believe the decrease will become greater (exponential) over the next few years.
I think it's going to reach a point that a snowball or cascade effect will occur. I think the org fears going back to meeting in person because it's not going to be the same... the halls will be emptier and the preaching work will be weak. That can only have a negative effect on JWs. That effect could be contagious. As more and more wake up and leave or just become inactive, the effect can spread rapidly (exponentially).
I've been monitoring exjw reddit and have found the number of JWs, especially young ones, leaving to be amazing. There are a lot of really insightful posts showing that these people have sized up the org, the religion, and the GB. They have seen through the deception. Young ones are not zealous and they are not deep-rooted in the org as the old-timers are, and the old-timers (those late 50's and above who can remember the glory days of JWdom) are dying off rapidly (and some are waking up, too). Barely any new ones will come in from the field, and those who do will not be the kind that can support and sustain the org. They won't be like the intelligent ones who became JWs in, for example, the 70's & 80's.
It seems to me that the wind is not filling the sails of JWs anymore. JWism is a religion that is based on growth and a bold, strong preaching work. The growth is over and the preaching work is a faint, weak whisper. That will have to get a lot of JWs to start thinking. Also, the internet continues to destroy the org and the religion. And... as somebody already mentioned in this thread, the lawsuits are stacked up against the org.
That "something massive" you mentioned could just be a tipping point at which a cascade effect occurs. There must be frustration now among the rank and file. There must be lack of zeal and lack of excitement. If they go back to meeting in person, the decreasing numbers will become more obvious. If they don't go back to meeting in person, the effect of meeting online could continue to weaken zeal (lots of anecdotal evidence on exjw reddit that it's doing that). The tipping could be reached and there could be a mass exodus. To me, the future looks really bad for JWdom.