I don't know if you intended this Kevin, but I think you've just ruled out time travel from the future into the past. Imagine a portal opens in thin air, and on the other side is 20 years in the future, where a time machine has been invented and has created the portal. Yes, the state of the world in the future is caused by the state of the world in the present, but the portal is not entirely caused by things that happened previous to the portal. Part of the causal chain of events that lead to the portal opening is chronologically after the portal opened.You might come to understand truths that you are absolutely certain are true, such as the fact that all things are entirely caused to happen by things that have happened previously.
Anything that happens as a result of stuff coming back through the portal is not entirely caused by things that had happened previously. For example, if my future self comes back through the portal and bumps into your present self today, this was not entirely caused by things that happened previous to today. In part, it was caused by things that will happen in the future, such as my future self walking toward the portal, the time machine being invented, etc.
Yes, this kind of time travel seems unlikely, but nonetheless, it is a matter of empirical observation. I would second the hypothesis that you will not find any time portals to the future. However, if a time portal to the future did suddenly open, you'd have to discard the hypothesis and revise the theory that made the erroneous prediction. No amount of being sure the portal won't open will prevent it from opening if the laws of physics allow it. There's plenty of physics yet to be understood, and the possibility of time travel is open. I don't think you can be absolutely sure all things are entirely caused by things that have happened previously.