journey-on, very interesting analysis... and I think you may have a valid assumption. Here's why:
While it was not a Bible Study that started me thinking "something is not right", it was a return visit I had. It was the only door I ever went to that had a sign that said "No Solicitors -- UNLESS you're one of Jehovah's Witnesses or a Mormon".
That guy invited me and my daughter in. I still don't remember all that we discussed over our two or three visits, but I kept getting backed into scriptural corners I couldn't answer, and he asked questions that made me think and question. Hmmm... I still remember what neighborhood he lived in. Maybe I should stop in and thank him!
So, while I don't think the "preaching work" will end, per se, I think their MANNER of doing it might very well change.