Laverite I love your reply to them and I've copied it to hopefully use myself one day if you don't mind.
Your words will stick in their minds and every time they tell another JW about your reply maybe it will drive the splinter in deeper (Matrix movie quote: "You don't know what it is, but it's there, like a splinter in your mind, driving you mad").
About 10 years or so before I left the JWs I remember reading a short interview in a women's magazine over here in Australia about a 'sister' who had left the organization. I never rememberd why she left etc but I always remembered her words that the JW people were generally nice people but those at the top and the organization itself were evil. It was the first time I had ever read anything like that and heard the word 'evil' used to describe the org and of course I dismissed it as apostate ravings, but the words stuck in my mind and of course now I know they are true.
About 5 years before I left I was reading a book about history or something and for some reason the author brought up JWs and said it was a mind controlled religion. Again this was a big shock to read this and I remember turning to my husband and asking increduously "are we mind controlled?". Of course he said no and we agreed it was just hateful propaganda/persecution. But again those words "mind-controlled" stuck in my brain like that 'splinter in the mind' and of course by the time we left we knew that JWs do belong to a mind controlled religion.
So hopefully laverite, your words "apocalyptic doomsday cult" and "100% failure rate in all its propecies" will be the splinter in their minds.