The onus of proof is usually on the person bringing the accusation. So they need to prove the truth when witnessing, but it is on the "apostate" when confronting a JW.
A JW would say they have 100 years of publications that outlines their proof for the truthfulness of their teachings. It seems that whilst I get your point, it would not make any difference to a JW. It is up to you to prove them wrong, and it takes a lot of work to get through the indoctrination.