Reform the disfellowshipping policy and everything will change. The threat of shunning is currently the evil force that holds the whole thing together.
"Just leaving" is simply not an option for many people because of the huge family and social implications, problems completely manufactured by the Watchtower Society.
So....... if shunning were to end (or at the very least, shunning for stopping being a JW, or what they call "apostasy") then yes, a huge percentage would leave, I think.
Reformists could sell the three-fold advantage to the org:
1. Sure, many will leave, but those who stay will be there out of free will and not fear, leading to a healthier religion.
2. "Apostates" and opposers will bug them less.
3. Divided families will be healthier once they're allowed to communicate once again.