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Here's my take:
Throughout his life, Willy Loman created great pain for his wife and children by living a life in which he did not realize what he could and couldn't do, and thus caused his children to lose their identity.
(I think saying he threatened their future is a little strong...)
OH, as we just discussed in the chat room:
What happens when you try to live the American Dream, but you fail? By not realizing what he could and couln't do, Willy Loman caused a great deal of pain to himself, his wife, and his children.