A more realistic scenario is that numbers will be fairly stagnant for a number of years, with some growth in poorer catholic countries and countries with high population growth (eg Africa), but in Western countries the average age of the rank and file member will continue to rise.
My speculation/hope from then would be the following:
In a decade or so, after a few years of stagnation, the baby boomers will start to die off, and decline will be visible to even the most ardent. Some ardent believes might think it means, "we must be so close", but as each year brings in further stagnation and decline, that would wear off.
In the meantime, Watchtower would be too preoccupied with financial issues and financial survival to manage the situation. With no clear leader, it may oscillate between being a little more liberal and being more hard-line. With the network of Kingdom Halls becoming more sparse, not all dubs will be able to attend their bi-weekly brainwashing session, or socialise exclusively with fellow dubs. This would make shunning more difficult and less effective as well.
A further decade along, the Borg would be in a clear downward trajectory. I would probably continue on for another 100 years or so, with a "quadruple-somersaulting double-overlapping pike-position generation" concept, before fading into oblivion.
Anyway, that is my hope.