If the future is open, it is never true that one is saying the truth
Given an open future, it is only true once one has already spoken that one says/said a particular sentence. For as long as one is still speaking, there is no fact of matter about how the sentence will end. It might end in an emphatic "--not!" So no utterance is true while it is being made. It can only be true after a decent pause. This is implausible, so we should reject the open future.