Extending it won't encourage the dead author to produce more work, but it can (theoretically) encourage people who are alive now to create works they wouldn't otherwise have done because they'll be able to better provide for their descendants.
Are the laws designed to fulfill this theoretical justification? Do they say that copyrights are extended for extremely long terms on the condition that the copyright remain with descendants, and pays them revenue?
No, they do not. If a corporation acquires a copyright then they receive any revenue, not descendants who may exist. Laws must be designed to fulfill the purpose that is used to justify them, otherwise you are just pulling arguments out of a hat.