The latest broadcast message from Anthony McCallan, er, Morris, insinuates that Zoom meetings will continue for another full year, minimum, and he drops the possibility it could extend even longer (he specifically mentions "2021 or even beyond").
That's 1-2 years, or maybe even longer, of electronic meetings and no door-to-door or cart witnessing.
I think financially they could continue this model indefinitely, but they aren't in it just for the money. More valuable to them is the control the exercise over, and the adulation they receive from, their adherents.
You can't get that with no elder visits and no meetings at a central location, so eventually they will return to something like they did before. The questions will be (1) how many of the post-COVID changes will stick as permanent and (2) how do you motivate JWs used to years of "letter writing in your pajamas" counting as "field service" to get back to dressing up and trudging around to knock at empty houses while either staining their dress clothes with sweat or subjecting their toes to frostbite?