Why the uncertainty about timing of collapse?
Simply because we don't know how much money Watchtower has on hand. We do know they have a revenue problem and there is no apparent solution in sight. So on this trajectory they will run out of money at some point. How long this will take depends on how much money they have in reserve and how skilful they are at managing decline. These things are not easy to determine.
Of course there is also the possibility they will reverse the decline. It's just not easy to see how.