Just to clarify:
I don't think "the ministry" will ever end. It is far too important for JW hierarchy. It is a practically perfect form of "busy work" that provides an easy to understand numerical representation to measure someone's "spirituality". In WT-world, a person who accomplishes "70 hours" is, by definition, always, without exception, considered more spiritual than someone who accomplishes only "20 hours". The 20-hour guy is, without exception, viewed as "more spiritual" than the 5 hour publisher, etc. The lowest of the low are the "inactive", the 0-hour guys.
Measuring "how spiritual" someone is, and using that as the basis for judgements, rewards, praise, granting of "privileges", etc. is the fundamental core, the soul, of what being a JW is.
So I don't think "the ministry" will end.
What I do anticipate is a great shift in emphasis, away from door to door, and more toward carts, letters, and telephoning. I think it i quite possible, perhaps even probable, that nearly all JW "ministry" will be something other than door to door, save for a few uber-zealous guys, and the ones who live out in the boonies where there is just no place to reasonably place a "literature" cart.