To be fair JWs don't ever actually say that only JWs will be able to live forever on earth. That being said it is HEAVILY implied.
Reasoning:
Only righteous will inherit the earth- to be righteous you must be in close association with God's spirit directed organization. They say that they are said organization. In fact when you are baptized as a JW you are baptized in the name of Jehovah, Jesus, and God's spirit directed organization.. lol WOW!
To be elegible for everlasting life you must be involved in righteous works- they say that based on scriptures that they are "spearheading" the preaching of god's kingdom today and no other religion does that. God's spirit directed organization is organizing this work today. Guess who..
I could go on and on but whats the point. They don't teach that only JWs will survive armageddon but they do. Go figure that one out.