So perhaps
1 - the future in which we have not had a visitor is just one future, there are others
2- it will be only possible to travel back in time to the moment that time travel became possible
So in 2- if the time machine was invented in 2020, then a person from 2040 could only get back to 2020, and not before.
??
Meanwhile, back on topic.
What I was referring to is not that there are various futures, but that if any future could be foreseen, or exist, then it must be pre-ordained.
Which I don't ascribe to.
But, all and any decision taken by anyone, from this moment forth, creates the future, and until those decisions are taken,
ad infinitum, the future is indeterminate.
As no-one can forsee the decisions, then no-one knows what the future is. In addition, any change of any decision, for any reason, will alter the future.
Similarly, any visit from the future will obviously alter the future, potentially preventing the visit, or altering the visit in some way, thereby creating a different future,
ad infinitum.
Thus, the future cannot exist, yet.