I saw or read something on sorta related a while back (I think it was a TED talk, but I can't find it) that discussed groups in which it is possible for everyone in the group to have serioius misgivings about whatever the group is doing, but everyone keeps it to themselves because there's a perception by all that everyone else is firmly in favor of the group. It's possible, at least in small pockets of the JW community, that this could be the state of affairs - everyone (or a large minority) going along because they think everyone else believes it. In such a pocket, a relatively small walk-out would have far-reaching effects.
That said, I don't think a grand appostate walk-out would severly criple the organization. Any positive feedback created would likely be isolated in small groups, and for everyone outside that group it would be easily explained away as a fluke, not part of a grander pattern. I've noticed in other threads that while 2/3rds of born-ins leave it seems that in many cases groups leave or stay together, for the most part. So there's many people who relate that all their friends from childhood are out, and many who say that everyone is still in.
This cult does seem to be dying a slow death in developed countries, though, so maybe in a few years such a walkout could happen and have a sweeping effect on the membership.