I've never had any success reasoning with JWs, and I don't expect to. They are very defensive about it. Somewhere deep inside, they might really know something's not right, but they throw up the defense out of fear.
I recently heard bridesmaids were also from a pagan origin, too. They were used to ward off an attack from evil spirits. If the evil spirits tried to attack the bride, they would get the bridesmaid instead. So, when are they going to ban bridesmaids from the weddings, I wonder?
If they are going to use the pagan origin thing they shouldn't be so selective about it. But being selective is really what they are all about. Selective scriptures that support whatever BS they are going on about, selective destruction by God, selective stories of being saved by God for going to the meetings during natural disasters, etc.