In trying to resolve the issues created by replacement theory, some assert that once the past has been changed it remains in its new form unless someone else from the future changes it to something else. This is a popular interpretation of Niven's Law, which asserts that in any universe in which history can be changed time travel will never be discovered, and is taken to mean that once the people in the future can fix whatever is wrong in the past, they will eliminate their need to create time travel because they have created the perfect past.
There are several problems with this, including that it ignores the desire to create time travel solely for the sake of knowledge. The one that matters most, though, is the issue of the causal chain. If Traveler goes back from 2020 to 2000 to change something and succeeds and returns to 2020, then his duplicate has no reason to make that trip and will not leave 2020 for 2000. This theory here says that this is not a problem because once the Traveler arrives in the past, he is part of the past, and he does not need to leave the future "again" to be in the past, as he is already there. Yet what if Traveler does leave from 2020 to travel to 2000, perhaps to change history from what it is (which he created) to what he does not know it originally was? Are there two versions of Traveler in the past, or only one? If there is only one, which future is he attempting to create? Thus this approach suffers from a problem similar to that of divergent dimension theory, only more so: if the duplicate traveler does not depart from the future, he is still present in the past, but if he does depart from the future it makes no sense either that he does or that he does not meet himself in the past.
The only logical way for the causal chain to play requires that if the traveler made a trip to the past, the traveler's duplicate must make the same trip to the past for the same reason with the same knowledge and abilities. If he does not do so, he erases his presence in the past. This then undoes the changes he made and restores the original history, which is how we get an infinity loop.
It has previously been noted that this interpretation of Niven's Law is not necessarily what Niven meant. He may have meant merely that free will is incompatible with the Novikov self-consistency principle, and therefore time travel will be impossible in any universe in which people have free will to change history. It is again wishful thinking; there is no evidence that dangerous things are impossible. However, this raises the issues of free will and randomness, our topic next time.